GEOLOGY   OF   TO-DAY 


— 


STATUE  OF  AGASSIZ  THROWN  FROM  ITS  NICHE  ABOVE  ARCHES, 
STANFORD  UNIVERSITY 

The  inversion  of  the  statue  indicates  its  direct  upthrow,  like  the  spring  of  a  diver. 


GEOLOGY   OF   TO-DAY 

A  POPULAR  INTRODUCTION  IN 
SIMPLE    LANGUAGE 


BY 


J.    W.    GREGORY,    F.R.S.,    D.Sc. 

PROFESSOR  OF  GEOLOGY  AT  THE  UNIVERSITY  OF  GLASGOW 

AUTHOR    OF 
:THE     GREAT     RIFT    VALLEY,"     "THE     DEAD     HEART     OF     AUSTRALIA,' 


r 

WITH  58  ILLUSTRATIONS  &  DIAGRAMS 


LONDON 

SEELEY,  SERVICE  6-  CO.  LIMITED 

38  GREAT  RUSSELL  STREET 

1915 


THE  SCIENCE  OF  TO-DAY  SERIES 

With  m*ny  illustrations.     Extra  Crown  Svff.     5*.  net. 

GEOLOGY  OF  TO-DAY  A  Popular  Introduction  in  Simple 
Language.  By  J.  W.  GREGORY,  F.R.S.,  D.Sc.,  Professor  of 
Geology  at  the  University  of  Glasgow,  Author  of  "  The  Dead 
Heart  of  Australia,"  &<:.  &c.  With  58  Illustrations.  55.  net. 

SUBMARINE  ENGINEERING  OF  TO-DAY.    A  Popular 

Account  of  Submarine  Engineering.     By  C.  W.  DOMVILLE- 

FIFE,  Author  of  "Submarines  of  the  World's  Navies,"  <SrV. 

55.  net 

BOTANY  OF  TO-DAY.    By  Prof.  G.  F.  SCOTT  ELLIOT,  M.A., 

B.Sc.,  F.L.S.    55.  net. 

"One  of  the  books  that  turn  botany  from  a  dryasdust  into  a  fasci- 
nating study." — Evening  Standard. 

AERIAL  NAVIGATION  OF  TO-DAY.  By  C.  C.  TURNER. 
"Mr.  Turner  is  well  qualified  to  write  with  authority  on  the  subject. 
The  book  sets  forth  the  principles  of  flight  in  plain  non-technical  language. 
One  is  impressed  by  the  complete  thoroughness  with  which  the  subject  is 
treated."— Daily  Graphic. 

SCIENTIFIC  IDEAS  OF  TO-DAY.  A  Popular  Account, 
in  Non-technical  Language,  of  the  Nature  of  Matter,  Elec- 
tricity, Light,  Heat,  Electrons,  &c.  By  C.  R.  GiBsON.F.R.S.E. 
55.  net. 

"Supplies  a  real  need.  .  .  .  Mr.   Gibson  has  a  fine  gift  of  exposi- 
tion."— Birmingham  Post. 

ASTRONOMY  OF  TO-DAY.     A   Popular   Introduction  in 
Non- technical  Language.     By  CECIL  G.  DOLMAGE,  LL.D., 
F.R.A.S.     With  46  other  illustrations.     Ex.  Cr.  8vo,  53.  net. 
11  Dr.  Dolmage  has  absolutely  kept  to  his  promise  to  introduce  the 
reader  to   an   acquaintance  with   the   astronomy   of  to-day  in   non- 
technical language." — Saturday  Review. 

ELECTRICITY  OF  TO-DAY.     Its  Work  and   Mysteries 

Explained.    By  C.  R.  GIBSON,  F.R.S.E.     55.  net. 
"  Mr.   Gibson  has  given  us  one  of  the   best   examples   of  popular 
scientific    exposition    that   we   remember    seeing.     His   book   may   be 
strongly  commended  to  all  who  wish  to  realise  what  electricity  means 
and  does  in  our  daily  life."—  The  Tribune. 

ENGINEERING  OF  TO-DAY.     By  T.  W.  CORBIN.    With 

73  illustrations.    55.  net. 

"The  description.s  which  are  given  of  various  types  of  engineering 
structures  and  work  are  excelieqt.'l— '-Yorkshire  Observer. 

MEDICAL  SCIENCE  OF  TO-DAY.     A  Popular  Account 
of  the  recent  Peye'.o.prnea.ts   in   Medicine  6*  Surgery.      By 
WILLMOTT   EVANS,    M.D.,:  B;St.-.    Ro'yal    Free    Hospital. 
55.  net. 
"A  very  Golconda  of  gems  of  knowledge."— Manchester  Guardian. 

MECHANICAL    INVENTIONS    OF    TO-DAY.      By 

T.  W.  CORBIN.     55.  net. 

"  In  knowledge  and  clearness  of  exposition  it  is  far  better  than  most 
works  of  a  similar  character  and  aim."— Academy. 

PHOTOGRAPHY  OF  TO-DAY.  A  Popular  Account  of 
the  Origin,  Progress  and  Latest  Discoveries  in  the  Photog- 
rapher's Art,  told  in  Non-technical  Language.  By  II. 
CHAPMAN  JONES,  F.I.C.,  F.C.S.,  F.R.P.S  ;  President  of  the 
Royal  Photographic  Society.  Extra  Crown  8vo,  55.  net. 

SEELEY,  SERVICE  &•  CO.   LIMITED 


' 


PREFACE 


THE  Geology  of  To-day  !  The  title  appals  me.  Modern 
geology  is  the  result  of  more  than  a  century's  work  by 
innumerable  and  indefatigable  pioneers  working  in  all 
parts  of  the  earth.  Their  discoveries  and  researches  are 
written  in  all  the  languages  of  the  civilized  world,  and 
geological  literature  is  so  voluminous  that  no  one  can 
read  it  all.  The  Geological  Society  of  London — the 
parent  of  geological  societies — publishes  an  annual  index 
of  the  literature  added  to  its  library;  the  index  for  19 n 
included  literature  published  in  582  magazines,  journals, 
and  other  serials,  and  a  total  of  over  2,500  books,  papers, 
and  memoirs.  A  geologist  would  have  to  read  eight  works 
a  day  to  keep  up  with  current  literature.  Such  a  task  is 
impossible ;  and  it  would  be  wasted  labour,  for  a  man  who 
attempted  it  would  have  no  time  to  use  the  knowledge  thus 
laboriously  acquired.  A  geologist  can  only  hope,  by  aid 
of  annual  indexes  and  catalogues  of  new  literature,  to  find 
the  contributions  to  his  own  special  lines  of  research ;  he 
will  have  to  acquire  a  slight  acquaintance  with  the  main 
conclusions  of  the  science  as  a  whole  through  reviews, 
summaries  of  progress,  and  conversation  with  his  col- 
leagues. A  full  digest  of  modern  geology  would  be  impos- 
sible in  the  space  of  a  book  of  this  size,  and  it  would  also 
be  comparatively  useless  to  a  general  reader.  Much 
geological  work  is  of  only  local  and  temporary  interest. 
Many  geologists  are  making  geological  bricks,  some  of 

vii 

346818 


Preface 

which  will  be  used,  while  others  may  lie  neglected  on 
the  field  where  they  were  made,  or  be  abandoned  on 
some  scientific  wayside.  Works  on  architecture  describe 
the  established  styles  of  buildings,  but  do  not  deal  with 
brick-making  and  cement-making;  similarly,  a  sketch 
of  current  geology  need  not  refer  to  the  work  of  all  the 
geological  pioneers  and  roadmakers,  but  it  should  state 
the  general  results  of  contemporary  work  and  indicate  the 
lines  which  are  to-day  of  especial  interest  and  promise  for 
the  progress  of  geologic  thought. 

The  reader  who  does  not  understand  the  technical 
language  used  by  the  actual  craftsmen  must  accept  several 
limitations  in  his  course  of  study.  Some  branches  of 
geology  deal  with  such  unfamiliar  ideas  and  materials 
that  they  cannot  be  considered  without  the  use  of  many 
technical  terms,  and  of  more  preliminary  explanation  than 
can  be  given  in  such  a  treatise  as  this. 

In  the  opening  up  of  a  new  country  provisional  tracks 
are  flung  boldly  forward  in  various  directions ;  later  on 
some  of  these  are  improved  into  permanent  roads,  while 
others  are  abandoned  as  better  routes  are  discovered. 
Scientific  progress  requires  many  provisional  hypotheses 
which  are  replaced  by  well-established  theories  and  laws 
as  the  basic  facts  are  better  understood.  A  visitor  to 
a  new  colony  generally  finds  it  more  profitable  first 
to  follow  the  main  routes  and  visit  the  settled  areas ;  he 
thus  travels  easily,  learns  the  mature  and  stable  views 
of  the  young  community,  sees  the  wider  issues  which  it 
has  to  determine,  and  hears  its  ideas  expressed  in  language 
which  he  can  readily  understand.  Thus  prepared,  he  can 
more  profitably  visit  the  backblocks  and  the  backwoods, 
see  the  pioneers  at  work  in  their  specialized  and  limited 
tasks,  and  understand  their  technical  language  or  local 
dialect. 

In  the  same  way  a  reader  who  wishes  to  understand  the 

viii 


Preface 

general  conclusions  respecting  the  history  of  the  earth  will 
profit  more  from  a  summary  of  ascertained  facts  than  from 
the  discussion  of  those  provisional  suggestions  which  are 
really  knowledge  in  the  making.  He  will  be  willing  to 
accept  the  limitation  involved  in  the  exclusion  of  technical 
language,  and  will  be  content  with  the  omission  of  un- 
familiar topics. 

These  limitations  render  it  advisable  to  exclude  or  give 
but  brief  consideration  to  various  problems  of  great  current 
interest.  For  example,  many  most  interesting  suggestions 
have  been  made  to  explain  mountain  formation  and  the 
origin  of  the  internal  heat  of  the  earth  by  the  phenomena 
of  radio-activity.  The  nature  of  radium  and  the  methods 
by  which  radio-activity  are  measured  can  only  be  explained 
by  the  introduction  of  advanced  physics  which  would  take 
much  space  to  interpret.  Moreover,  the  literature  of  the 
subject  during  the  past  few  years  has  been  marked  by  so 
many  rapid  changes  of  view,  and  many  of  the  results  are 
so  inconsistent,  that  the  popular  discussion  of  their  bear- 
ings on  geology  may  be  delayed  until  the  physicists  have 
reached  some  more  definite  conclusions  about  the  facts  of 
radio-activity  which  bear  upon  geological  problems.  The 
contributions  to  radio-active  geology  have  already  been 
very  helpful ;  but  this  new  branch  of  science  is  still  so 
speculative  that  geologists  may  be  excused  from  any 
immediate  modification  of  their  fundamental  principles. 


IX 


AUTHOR'S  NOTE 

THE  author  would  like  to  acknowledge  the  kindness  of 
the  following  for  permission  to  use  illustrations  supplied 
by  them :  Mr.  H.  R.  Knipe  for  pictures  of  antediluvian 
animals ;  Mr.  Carl  Hagenbeck  for  photographs  of  the 
reproductions  of  antediluvian  animals  in  his  Zoological 
Park  at  Hamburg  ;  the  Carnegie  Institute  of  Washington 
for  photographs  of  the  San  Francisco  earthquake  of  1906 ; 
and  the  Yerkes  Observatory  for  an  astronomical  photo- 
graph. 


CONTENTS 


PREFACE         -                 ...                 .  .                          vil 

PART  I 

INTRODUCTORY 

CHAPTER 

I.  THE  DEVELOPMENT  OF  MODERN  GEOLOGY  -           17 

II.  THE  BIRTH  OF  THE  EARTH  -  -    35 

HI.  THE  GEOLOGY  OF  THE  INNER  EARTH  -    52 

IV.  THE  MATERIALS  OF  THE  EARTH'S  CRUST  -           6 1 

PART  II 
PHYSICAL  GEOLOGY 

V.    THE  WEARING  DOWN  AND  UPLIFTING  OF  THE  EARTH'S 

CRUST     -  -         88 

VI.    FOLDS   AND    FAULTS  -  -         94 

VII.    EARTHQUAKES  -         98 

VIII.    VOLCANOES    -  Il8 

IX.    HOW   MOUNTAINS    ARE    MADE  -       142 

X.    HOW   MOUNTAINS    ARE    UPHELD  -       155 

PART  III 
HISTORICAL  GEOLOGY 

XI.    THE   AGE   OF   THE   EARTH       -                 -  -       1 66 

XII.   THE   ERA    OF   THE   DAWN    OF   LIFE   (EOZOIC)  -                 -       189 

XIII.  THE  FISH  ERA  OR  ERA  OF  ANCIENT   LIFE  (PALEOZOIC)       198 

XIV.  THE   REPTILE   ERA   (MESOZOIc)  -       213 
XV.    THE    MAMMAL    ERA    (KAINOZOIC)  -       219 

xi 


Contents 


PART  IV 
THE  STORY  OF  LIFE  ON  THE  EARTH 

CHAPTER  PAGE 

XVI.   THE   ORIGIN   OF   LIFE  —  *  -  233 

XVII.    THE  INTERPRETATION   OF    FOSSILS  :    FOSSIL   FISH    AND 

AMPHIBIANS          -  -  239 

XVIII.    THE   ANCIENT   REPTILES   AND   THE   ORIGIN   OF    BIRDS  249 

XIX.    THE   EVOLUTION   OF   MAMMALS  -  266 

XX.    THE   DRIVING   POWER   OF   EVOLUTION  -  284 

XXI.    THE   SIZE   OF   EXTINCT   ANIMALS          -  -  294 

XXII.    THE   GEOLOGICAL   HISTORY   OF   MAN  -  -  -  300 

INDEX  -  -  -  -  -  -  324 


Xll 


LIST  OF  ILLUSTRATIONS 


INVERSION   OF   A   STATUE     - 

THE   GREAT   NEBULA   OF   ORION 

A    ROAD   NEAR   WRIGHT,    CALIFORNIA 

A   WRECKED   LIBRARY 

TWISTED    MONUMENTS 

VOLCANIC   CONE   OF   SMERO 

A    MUD   STREAM 

TRAIN-RAILS    BUCKLED    BY    EARTHQUAKE      - 

A    SMALL   CRATER    - 

CRATER   OF    BROMO,   JAVA    - 

WITHIN   THE   CRATER    OF   BROMO     - 

THE   VOLCANOES   OF   JAVA    - 

THE   SUMMIT   OF    KLOET,    JAVA 

HILLS   IN   THE   VOLCANIC   REGION    OF   JAVA 

PERMIAN    REPTILES 

IGUANODON 

FLYING   REPTILES     - 

A    CERATOSAURUS    - 

TRICERATOPS 

GIGANTOSAURUS 


Frontispiece 

FACING  PAGE 

-  38 

-  100 
IOO 

-  108 

-  120 

-  122 

-  126 

-  126 

-  128 
'   132 

!34 

-  138 
148 

-  2IO 

-  2l6 
246 

-  254 

-  256 


xiii 


List  of  Illustrations 


PAGE 
-         260 


A    PLESIOSAURUS 

A    MEGATHERIUM      -  *  280 

MACHRAUCHENIA     -  -  288 

THE  PTERANODON  -  -  296 

FLINT    IMPLEMENT  -  3°6 

NEOLITHIC   FLINT   IMPLEMENTS  -                 -                 *     '            -  308 


LIST  OF  DIAGRAMS 

FIG.  PAGE 

1.  DIAGRAM    OF   A   DISSECTED   HIGHLAND  -  23 

2.  DIAGRAM    ILLUSTRATING    THE    RISE    OF   THE    SEA    UPON 

THE  LAND  IN  CONSEQUENCE  OF  THE  UPRISE  OF  THE 
SEA-FLOOR  -  33 

3.  DIAGRAM      ILLUSTRATING      THE     SHALLOW     ORIGIN     OF 

EARTHQUAKES  -          57 

4.  DIAGRAM    ILLUSTRATING   THE   GREATER   LENGTH    OF   AN 

EARTHQUAKE  ROUTE  ALONG  THE  ARC  OBC  THAN 
THROUGH  THE  EARTH  STRAIGHT  FROM  O  TO  C  -  58 

5.  DIAGRAM   EXPLAINING  THE   QUICKENING  OF   AN   EARTH- 

QUAKE WAVE  ON  ITS  ROUTE  FROM  O  TO  D  OR  E, 
OWING  TO  ITS  PASSAGE  THROUGH  THE  GEITE  CORE 
OF  THE  EARTH  -  59 

6.  DIAGRAMMATIC     LONGITUDINAL     SECTION     OF    A     SHEET 

OF    IGNEOUS    ROCK    AT    LUGAR,    AYRSHIRE    (TYRRELL)         83 

7.  SECTIONS   OF    FOLDS       -  -         95 

8.  SECTIONS   OF   FAULTS     -  -         96 

9.  DISTRIBUTION    OF    EARTHQUAKES    AND    VOLCANOES      >  -       103 

10.  THROW   OF   THE    KHASI    MONUMENTS      -  .     ^       107 

11.  LINE   OF   THE   FAULT   NEAR   SAN    FRANCISCO       -  ,    '*•      1 1 1 

xiv 


List  of  Illustrations 


FIG.  PAGE 

12.  DISPLACEMENT    OF    ROAD    BY    A    FAULT    AT    MINO-OWARI 

EARTHQUAKE                                                     T                                     -  112 

13.  DISPLACEMENT    OF    FENCE    BY    EARTHQUAKE        -                  -  112 

14.  COURSE    OF   THE    EARTHQUAKE    LINE     ALONG    WESTERN 

AMERICA       -                                                                                         -  113 

15.  DISTRIBUTION     OF     SOME     BRITISH     EARTHQUAKES     BE- 

TWEEN  THE   YEARS    1884   AND    1910                                   -  JI5 

1 6.  THE    BAY    OF    NAPLES     -                                                                           -  II'J 

17.  SECTION    ACROSS   THE   GREAT   GEYSER   OF   ICELAND          -  122 

1 8.  SKETCH-MAP   OF   WEST    INDIES  -                                                     -  1 29 

19.  OUTLINE   OF   THREE   VOLCANIC   CONES   -                                   '  J33 

20.  DISTRIBUTION    OF    SOUNDS    AND    DUST    FROM    KRAKATOA 

ERUPTION     -  136 

21.  SECTION    ACROSS    THE    DEVIL'S    CANYON                                      -  139 

22.  RELATION    OF   VOLCANOES    TO    SINKING   OCEAN-FLOORS-  150 

23.  RELATION    OF    COASTAL    MOVEMENTS    AND    ROTATION    OF 

THE    EARTH                                                                                               -  153 

24.  ISOSTATIC    EQUILIBRIUM                                                                          -  l6l 

25.  ISOSTATIC    MAP    OF   THE    UNITED    STATES                                    -  164 

26.  ISOCHLOR    IN    SOUTHERN   SCOTLAND        -                                   -  177 

27.  FIGURE   STONES    FROM    PALAEOLITHIC   GRAVELS-                 -  301 

28.  COMPARATIVE      OUTLINES     OF     ANCIENT     AND      MODERN 

SKULLS           -                                                                                         *  3IO 

29.  OUTLINE    OF    SKULL    OF    A    NEANDERTHAL    MAN                   -  311 

30.  RESTORED    OUTLINE   OF    PILTDOWN    SKULL                             -  312 

31.  OUTLINE    OF    SKULL    OF    A    MODERN    MAN                                  -  313 

32.  RELATIONS     OF     EVOLUTION     OF     MAN     FROM     MIOCENE 

MONKEYS      -                                                                                              -  317 


XV 


GEOLOGY    OF    TO-DAY 


PART    I 
INTRODUCTORY 

CHAPTER  I 
THE  DEVELOPMENT  OF  MODERN  GEOLOGY 

GEOLOGY  is  a  science  of  entirely  modern  growth.  Even 
the  name  is  comparatively  young,  for  geology  was  first 
proposed  by  Deluc  of  Geneva  in  1778.  Many  ancient 
and  classical  thinkers  speculated  on  the  origin  of  the 
earth,  and  though  some  of  them  drew  correct  conclusions 
as  to  the  geological  processes,  their  views  as  a  whole  were 
erratic  and  unscientific.  The  best  contributions  to  ancient 
geology  were  such  descriptions  of  materials  as  Theo- 
phrastus's  "  History  of  Stones  "  ;  and  considerable  know- 
ledge of  economic  geology  is  indicated  by  the  success  of 
ancient  stone -working  and  metal -mining,  though  no 
definite  written  records  of  the  methods  used  have  been 
preserved.  It  was  not  until  the  Italian  Renaissance  that 
geological  inquiry  began  on  modern  lines.  Medieval 
thinkers,  such  as  Dante,  speculated  on  the  interchange 
in  position  of  land  and  water,  and  practical  engineers, 
such  as  the  versatile  Leonardo  da  Vinci,  recognized 
ancient  sea-shells  in  rocks  and  their  meaning ;  and  as 
the  evidence  of  these  fossils  bore  directly  on  the  pre- 
historic history  of  the  earth,  it  at  once  came  in  contact 
with  theology.  The  study  of  geology  was  pursued 

17  B 


Dey.elppment  of  Modern  Geology 

mainly  from  its  bearing  on  the  Mosaic  account  of 
the  origin  of  the  earth,  and  the  early  schemes  of 
geology  were  cosmogonies,  such  as  Bishop  Burnett's 
"  Sacred  History  of  the  Earth,"  published  in  1684.  It 
was  not  until  the  later  part  of  the  eighteenth  century 
that  these  early  studies  led  to  the  development  of  better 
methods  of  inquiry ;  and  as  the  facts  discovered  were 
inconsistent  with  the  Mosaic  account  of  the  Creation,  the 
universality  of  Noah's  Deluge,  and  the  literal  interpreta- 
tion of  the  Bible,  there  was  a  furious  controversy  between 
the  theologians  and  the  geologists. 

"Geology  is  here"  (i.e.,  Italy),  says  Zangwill,  "the 
handmaiden  of  art  and  theology."  In  the  eighteenth 
century  it  was  expected  to  be  a  choir-boy  to  theology; 
and  as  it  preferred  secular  songs  to  sacred  hymns,  it  fell 
into  dire  disgrace. 

Geology  deals  with  the  earth  as  a  whole,  whereas  at  that 
time  many  men  were  more  interested  in  the  creation  and 
origin  of  the  earth  than  in  the  later  stages  of  its  history. 
They  were  therefore  cosmogonists,  and  not  geologists. 
As  geology  rejected  the  cosmogony  taught  by  current 
theology,  many  geologists  were  naturally  tempted  to 
advance  a  newer  cosmogony  of  their  own;  hence  they  were 
led  into  the  blunders  inevitable  from  trying  to  start  on  one 
of  the  final  problems  of  geology.  A  sound  geologic  cos- 
mogony was  then  unattainable  owing  to  ignorance  of  the 
elementary  facts.  It  was  impossible  to  interpret  the  very 
difficult  evidence  of  the  oldest  rocks  until  the  simpler 
younger  rocks  had  been  carefully  investigated.  The  few 
facts  known  were  linked  together  by  so  much  fiction  that 
the  cosmogonies  were  unscientific  and  absurd.  Even  the 
observations  were  not  reliable,  as  they  were  distorted  by 
a  cosmogonic  squint. 

It  had  been  recognized  that  many  rocks  were  formed  as 
deposits  laid  down  in  water,  such  as  the  gravels  laid  down 

18 


Development  of  Modern  Geology 

on  river-beds,  and  sheets  of  sand  and  mud  along  the  sea- 
shore. These  rocks  were  therefore  called  " aqueous" 
(Lat.,  aqua,  water).  Beds  of  sand  and  gravel  were  clearly 
formed  by  water,  and  many  rocks,  such  as  sandstone  and 
pudding-stone,  were  clearly  only  sand  and  gravel  hardened 
by  a  cement ;  and  other  rocks,  such  as  quartzite  and  slate, 
had  less  perfectly  preserved  evidence  of  their  formation 
by  water.  Hence  many  geologists  considered  that  those 
rocks,  such  as  granite,  which  had  no  trace  of  having  been 
formed  by  water,  were  really  aqueous  rocks  which  had 
lost  all  traces  of  their  origin.  Such  geologists  held  that 
all  rocks  were  originally  due  to  the  action  of  water ;  such 
men  were  therefore  called  Neptunists,  from  Neptune,  the 
God  of  the  Sea. 

Another  school  of  geologists,  however,  recognized  that 
many  rocks  were  formed  during  conditions  of  such  intense 
heat  that  they  had  been  molten,  and  cooled  either  as  beds 
of  lava,  when  they  had  been  discharged  to  the  surface  of 
the  earth  through  volcanoes,  or  had  solidified  in  great 
masses  below  the  earth's  surface.  These  geologists  were 
therefore  called  Plutonists,  after  Pluto,  the  God  of  the 
Infernal  Regions.  The  Plutonists  were  as  much  inclined 
to  exaggerate  the  extent  of  fire-formed  rocks  as  the  Nep- 
tunists were  those  due  to  water. 

The  controversy  between  these  two  schools  was  carried 
to  such  extreme  lengths  that  it  was  ridiculed  by  Goethe  in 
his  drama  of  "  Faust." 

The  Neptunists  used  some  very  plausible  mineralogical 
arguments  against  the  origin  of  granite  from  a  molten 
condition,  for  it  contains  minerals  which  could  not  exist 
at  a  high  temperature,  and  the  relations  between  the 
three  simple  minerals  of  which  granite  is  composed  are 
not  those  that  were  then  reasonably  expected  if  they  had 
formed  during  the  solidification  of  a  molten  granite ;  for 
the  mineral  quartz,  which  solidified  the  last  of  the  three, 


Development  of  Modern  Geology 

is  the  mineral  which  has  the  highest  melting-point,  and 
was  therefore  expected  to  solidify  first. 

Nevertheless,  when  the  relations  of  granite  to  other 
rocks  were  studied  on  the  field,  the  granite  appeared  as  if 
it  had  been  forced  when  liquid  into  older  rocks,  a  fact 
observed  by  Hutton  in  Glen  Tilt  in  Perthshire.  At  the 
time  there  was  no  means  of  reconciling ,  these  contra- 
dictory arguments,  and  it  was  not  until  long  afterwards 
that  it  was  learnt  that  the  argument  from  the  solidifying 
of  quartz  in  granite  after  the  other  constituents  was  based 
on  a  fallacy,  which  is  explained  on  p.  79. 

In  order  to  free  geology  from  hopeless  attempts  to 
solve  problems  which  could  not  be  solved  with  the  know- 
ledge then  available,  and  to  get  rid  of  the  incubus  of 
unscientific  and  premature  hypotheses,  a  group  of  British 
geologists  founded  the  Geological  Society  of  London. 
The  rash  pursuit  of  cosmogonies  did  not  suit  the  British 
mind,  which  preferred  facts  that  could  be  established 
by  observation  to  the  uncertain  products  of  speculation. 
Mephistopheles,  in  Goethe's  "  Faust,"  speaking  as  the 
evil  genius  of  Continental  science,  sneers  at  the  British 
love  of  facts : 

"  Are  Britons  here  ?    They  travel  far  to  trace 
Renowned  battlefields  and  waterfalls." 

Further,  the  facts  collected  by  early  British  geologists 
were  facts  of  practical  value.  They  preferred  economics 
to  cosmogonies.  Thus  Lister  in  1684  studied  the  distri- 
bution of  rocks  ;  Strachey  in  1719  and  1725  wrote  on 
mining  geology  ;  the  British  Board  of  Agriculture  in  1794 
began  agricultural  surveys ;  and  William  Smith,  the 
father  of  geology,  who  was  an  enginer  and  surveyor, 
lamented  "  that  the  theory  of  geology  was  in  possession 
of  one  class  of  men,  the  practice  with  another."  This 
division  of  interest  of  that  time  was  really  fortunate. 

20 


Development  of  Modern  Geology 

The  subject  was  so  complex  that  a  sound  science  of 
economic  geology  was  impossible  until  theoretical  geology 
had  made  great  progress,  and  the  work  then  of  most 
permanent  value  was  that  pursued  in  a  purely  academic 
spirit.  It  was  carried  on  in  spite  of  the  insinuation  that 
geology,  after  having  been  a  dangerous,  was  becoming  "  at 
best  but  a  visionary  pursuit." 

The  visionaries  in  1807  founded  the  Geological  Society 
of  London  to  reform  the  methods  of  geology.  They  were 
full  of  contempt  for  the  vain  wranglings  between  Nep- 
tunists  and  Plutonists.  The  Geological  Society  was  in- 
tended to  divert  geology  from  such  controversies,  and  to 
direct  geological  inquiry  into  useful  channels.  Its  aim 
was  to  secure  a  foundation  of  positive  knowledge  on  which 
future  generations  could  establish  firmly  a  full  geological 
system.  This  was  proclaimed  in  1811,  when  the  Society 
adopted  as  its  motto  a  passage  from  Bacon,  which  recom- 
mended toil  instead  of  talk.  Its  loyalty  to  this  principle 
was  remarked  by  Fitton  in  1817,  who,  in  an  account  of 
the  Society's  Transactions  in  the  Edinburgh  Review,  said 
that  they  were  limited  to  the  record  of  "  strict  experiment 
or  observation,  at  the  expense  of  all  hypothesis,  or  even 
of  moderate  theoretical  speculation." 

Lyell  in  1832  expounded  the  ideal  of  the  founders  as 
being — "  to  multiply  and  record  observations,  and  patiently 
await  the  result  at  some  future  period ;  and  it  was  their 
favourite  maxim  that  the  time  was  not  yet  come  for  a 
general  system  of  geology,  but  that  all  must  be  content 
for  many  years  to  be  exclusively  engaged  in  furnishing 
materials  for  future  generalizations  ";  and  he  claimed  that 
the  Society  had  secured  brilliant  success  as  the  reward  of 
its  consistency  to  these  principles. 

From  1807  began  the  active  progress  of  modern 
geology;  the  Neptunists  and  Plutonists  were  found  to 
be  both  partially  right  and  partially  wrong,  and  geology 

21 


Development  of  Modern  Geology 

was   gradually  established   on  a   firm   basis  of  carefully 
observed  and  verified  facts. 

The  main  cleavage  among  the  geologists  in  the  earlier 
part  of  the  nineteenth  century  was  between  the  two  schools 
known  as  the  Catastrophists  and  the  Uniformitarians. 
The  Catastrophists  were  so  called  because  they  attributed 
many  features  on  the  earth's  crust  to  great  catastrophes. 
It  was  known  that  many  cities  had  been  overwhelmed 
by  volcanic  explosions  and  others  had  been  destroyed  by 
earthquakes ;  and  during  these  world-shaking  earthquakes 
rocks  were  thrown  down  in  vast  landslips,  parts  of  the 
earth's  crust  were  upheaved  and  lowered,  and  parts  were 
rent  by  fissures.  The  older  rocks  were  found  to  be  traversed 
by  many  fractures  and  dislocations,  known  as  "  faults  " ;  the 
rocks  on  one  side  of  the  fault  had  been  raised  or  lowered, 
and  as  the  faults  may  extend  all  across  a  wide  stretch  of 
country  and  displace  the  rocks  beside  them  for  thousands 
of  feet,  the  faults  are  evidence  of  earthquakes  in  former 
geological  times.  Moreover  great  masses  of  bones  of 
extinct  animals  are  found  huddled  together ;  sometimes 
skeletons  are  found  entire,  and  at  others  there  are  collec- 
tions of  miscellaneous  bones,  which  appear  to  have  been 
swept  together  as  by  a  violent  flood.  One  school  of 
geologists  therefore  concluded  that  the  earth  had  in  the 
past  been  subject  to  geographical  catastrophes  similar  to 
to  those  which  have  been  experienced  in  historic  times, 
but  far  more  powerful  and  disastrous.  And  to  the  direct 
effect  of  such  catastrophes  they  attributed  the  formation 
of  valleys,  the  upraising  of  mountains,  the  lowering  of  the 
sea-floor,  and  the  repeated  extermination  of  whole  groups 
of  animals  and  plants.  According  to  this  school  the 
history  of  the  earth  was  a  record  of  slow  changes  due  to 
the  action  of  the  forces  of  the  air  and  of  the  sea,  and 
of  sudden  changes  due  to  the  convulsions  of  the  earth's 
crust. 

22 


Development  of  Modern  Geology 

The  Catastrophic  theory  was  opposed  by  the  geologists 
known  as  the  Uniformitarians,  of  whom  one  of  the  first 
was  Hutton  of  Edinburgh  ;  but  his  views  were  marred  by 
their  narrowness,  and  the  real  founder  of  this  school  was 
Charles  Lyell.  This  school  held  that  the  most  important 
agencies  in  moulding  the  earth  were  those  that  we  can  see 
everywhere  around  us  slowly  at  work  to-day.  The  rocks 
exposed  on  the  surface  are  quietly  undergoing  decay  owing 
to  the  chemical  action  of  the  air,  the  shattering  effect  of 
alternate  heating  in  the  day  and  cooling  at  night,  the 
wear  and  tear  of  dust,  wind,  and  water.  The  decayed  rock 
material  is  then  scattered  by  wind,  rain,  rivers,  and  tidal 
currents,  and  deposited  as  fresh  beds,  many  of  which  are 
ultimately  converted  into  rocks.  Valleys  are  seldom  formed 


FIG.  i.— DIAGRAM  OF  A  DISSECTED  HIGHLAND. 

by  violent  disruptions,  but  are  due  to  slow  processes,  and 
are  usually  worn  out  by  rivers.  Mountains  are  not  due 
to  sudden  upheavals,  but  to  the  slow  bending  of  the  crust, 
and  most  of  them  are  masses  left  as  highlands  by  the 
removal  of  the  surrounding  material.  Thus  the  mountains 
and  valleys  on  Fig.  i  were  not  formed  by  violent  move- 
ments of  the  crust,  but  by  a  plateau  having  been  carved 
into  the  valleys — v1  to  v5 — by  rivers  ;  and  the  intervening 
mountains  are  merely  the  ground  left  between  the  valleys, 
and  are  still  in  progress  of  reduction.  The  valleys  are 
valleys  of  erosion,  and  the  mountains  are  residual  moun- 
tains, as  they  are  the  residues  of  once  continuous  sheets 
of  rock. 

The  valleys  of  the  world  are  so  large  and  the  rivers 
appeared  so  relatively  feeble  that  it  was  natural  at  first  to 

23 


Development  of  Modern  Geology 

regard  the  rivers  as  formed  by  the  valleys  instead  of  the 
reverse.  The  study  of  rivers  and  the  careful  measurement 
of  their  action  show  that  even  quiet  rivers  wear  away  the 
land  beside  them  more  quickly  than  they  seem  to  do. 
Thus  the  Thames  is  a  comparatively  sluggish  river ;  but 
it  carries  to  the  sea  every  year  in  solution  material  sufficient 
to  form  a  block  of  limestone  as  large  as  St.  Paul's  Cathedral, 
and  it  sweeps  along,  as  particles  of  mud  and  silt  floating 
in  its  water  or  as  grains  of  sand  rolled  along  its  bed,  a 
still  larger  quantity  of  material. 

Other  rivers  are  far  more  active.  Alpine  torrents  wear 
away  their  beds  so  quickly  that  the  banks  fall  in  great 
landslips,  and  the  valleys  enlarge  far  more  in  a  year  than 
the  Thames  Valley  does  in  a  century.  The  Mississippi, 
after  prolonged  and  careful  measurements,  is  estimated 
to  lower  the  average  level  of  the  whole  of  its  vast  basin 
by  one  foot  in  6,000  years.  Hence,  given  enough  time,  the 
work  of  rivers  and  wind  is  quite  adequate  to  explain  the 
formation  of  the  largest  valleys. 

The  Uniformitarians  did  not  exclude  more  violent 
agencies.  Volcanic  eruptions  were  known  to  pile  up 
volcanic  mountains  by  successive  eruptions,  or  by  violent 
explosions  to  blow  whole  mountains  to  fragments  and 
scatter  the  material  widespread  over  the  surrounding 
country.  Earthquakes  were  known  to  tear  rocks,  over- 
throw cities,  and  suddenly  to  lower  or  raise  wide  tracks 
of  country.  The  influence  of  such  violent  geographical 
catastrophes  was  recognized,  but  it.  was  held  that  the 
earthquakes  and  volcanoes  of  the  past  had  been  of  similar 
power  to  those  of  modern  times.  Hence  it  was  believed 
that  the  existing  form  of  the  earth's  surface  was  due  to 
the  long-continued  action  of  the  same  forces  as  are  acting 
now,  and  that  they  always  worked  with  about  the  same 
intensity  as  at  present. 

The  fundamental  principle  of  the  Uniformitarian  school 

24 


Development  of  Modern  Geology 

as  expressed  in  the  words  of  its  great  teacher  Lyell,  is 
that  "  all  theories  are  rejected  which  involve  the  assump- 
tion of  sudden  and  violent  catastrophes  and  revolutions 
of  the  whole  earth  and  its  inhabitants."  Some  Uniformi- 
tarians,  it  is 'true,  went  much  further.  Thus  Hutton  of 
Edinburgh  maintained  that  all  valleys  were  due  to  the 
slow  excavation  by  rivers,  and  he  denied  that  any  of  the 
depressions  on  the  earth's  surface  were  due  to  the  sinking 
of  the  crust,  or  that  any  of  the  mountains  were  due  to 
upheaval ;  he  admitted  that  rocks  formed  below  the  sea 
were  upheaved  in  periodic  disturbances,  but  he  maintained 
that  the  existing  irregularities  were  carved  out  of  the 
upraised  masses  by  rivers. 

We  owe  mainly  to  Charles  Lyell  the  development  of 
a  system  of  geology  which  assigns  a  reasonable  share  to 
all  the  great  forces  that  affect  the  crust  of  the  earth.  He 
recognized  that  sudden  local  catastrophes  due  to  flood, 
earthquake,  and  volcano,  have  often  happened,  and  that 
they  had  probably  happened  in  the  past  on  a  greater  scale 
than  any  of  which  there  are  historic  records.  He  knew 
that  in  some  parts  of  the  earth  all  the  existing  hills  and 
valleys  were  due  to  a  slow  sculpture  of  the  land ;  but  he 
also  realized  that  in  other  cases  valleys  and  basins  are 
due  to  a  sinking  of  the  land,  and  that  mountains  or  plains 
are  directly  upraised  by  a  slow  bending  of  the  crust  or  by 
a  sudden  uplift. 

It  was  the  glory  of  Lyell  that  he  saw  the  earth  and 
the  forces  that  act  on  it  as  a  whole ;  he  had  none  of  the 
provincial  narrowness  of  many  of  his  predecessors,  who 
applied  to  the  whole  world  the  processes  that  were  most 
powerful  in  their  own  district,  and  rejected  forces  which 
had  there,  perhaps,  never  been  at  work,  or  were  at  least 
dormant. 

Lyell  recognized  that  different  parts  of  the  earth  are  at 
present  being  affected  by  different  geographical  processes  ; 

25 


Development  of  Modern  Geology 

in  one  place  rain  and  rivers,  in  another  movements  of  the 
crust,  and  in  yet  others  earthquakes  and  volcanoes,  are 
the  dominant  agencies,  and  that  at  different  times  the 
same  area  may  have  been  affected  by  different  agencies. 
The  British  Isles,  for  example,  have  now  had  a  long 
period  free  from  volcanic  disturbances,  but  at  different 
periods  in  geological  history  our  islands  have  been  the 
scene  of  most  powerful  volcanic  activity. 

The  Lyellian  doctrine,  therefore,  fully  recognized  the 
great  instability  of  the  crust  of  the  earth.  When  marine 
shells  were  found  above  sea-level  and  often  at  some  dis- 
tance from  the  sea,  it  was  natural  at  first  to  explain  them 
as  due  to  the  sea  having  formerly  extended  to  that  locality, 
and  having  since  receded.  The  land  appears  fixed  and 
stable.  The  surface  of  the  sea,  on  the  other  hand,  is 
always  in  motion.  Its  level  rises  and  falls  daily  with  the 
tide ;  powerful  storms  hurl  the  water  in  high  waves 
against  the  land ;  prolonged  steady  winds  cause  floods  by 
piling  up  the  water  in  estuaries,  or  they  cause  shallows  by 
blowing  away  the  water. 

Hence  the  former  extension  of  the  sea  over  the  land 
appeared  to  be  most  easily  explained  as  due  to  variations 
in  the  level  of  the  sea.  It  was  found,  e.g.,  that  the  Baltic 
once  covered  parts  of  Sweden ;  and  Celsius,  the  inventor 
of  the  Centigrade  thermometer,  early  in  the  eighteenth 
century,  concluded  that  the  waters  of  the  Baltic  and 
North  Seas  were  subsiding  about  forty  inches  in  a  century. 
It  was  replied  that  this  was  impossible,  as  the  sea-level 
could  not  fall  in  the  Baltic  and  North  Seas  without  a 
general  sinking  of  the  sea  surface  over  the  whole  globe ; 
and  there  is  conclusive  evidence  that  in  parts  of  the 
southern  Baltic  the  level  of  the  water  has  been  constant 
throughout  historic  times.  Thus  the  low  island  of  Salt- 
holm,  near  Copenhagen,  has  remained  at  the  same  level 
in  regard  to  the  sea  surface  since  at  least  the  year  1280, 

26 


Development  of  Modern  Geology 

and  the  water-level  in  ports  such  as  Dantzic  on  the 
southern  shore  of  the  Baltic  has  remained  the  same  since 
at  least  the  year  1000.  In  1807  von  Buch  reconciled  the 
two  series  of  facts,  for  he  showed  that  in  northern  Sweden 
there  is  clear  evidence  of  the  emergence  of  the  land  from 
the  sea ;  that  northern  Norway  has  emerged  from  the 
North  Sea,  but  to  a  less  extent ;  and  that  in  southern 
Sweden  there  has  been  no  such  change  in  the  relative 
levels  of  land  and  sea. 

As  it  was  held  that  the  average  of  the  sea  surface, 
ignoring  temporary  variations  due  to  waves  and  tides, 
must  be  level,  this  change  must  be  due  to  the  tilting  of 
the  land ;  and  von  Buch  therefore  concluded  that  most  of 
Scandinavia  is  rising,  and  that  the  rise  is  greatest  on  the 
shores  of  the  Gulf  of  Bothnia.  This  conclusion  has  been 
confirmed  by  later  observations,  and  the  cause  of  the 
movement  has  been  explained  (see  p.  161). 

Lyell  carefully  investigated  the  facts,  and  recognized 
the  slow  uprise  of  the  land.  In  many  cases  ii  is  clear  that 
the  land  has  been  raised  and  lowered  when  the  crust  was 
being  bent  into  great  folds.  Thus  the  geography  of  the 
south-east  of  England  has  been  largely  determined  by 
a  great  double  fold  of  the  crust  of  the  earth.  Formerly 
the  whole  area  from  Norfolk  to  northern  France  was 
occupied  by  a  continuous  horizontal  sheet  of  that  soft 
white  limestone  known  as  "  chalk."  This  sheet  of  chalk 
was  then  bent ;  a  downfold  formed  a  trough  along  the 
Thames  Valley  and  another  along  the  English  Channel, 
while  upfolds,  or  arches,  formed  the  hills  of  East  Anglia 
and  a  high  mountain  -  ridge  across  Kent  and  Surrey ; 
and  though  this  mountain-ridge  has  been  worn  away,  its 
existence  can  be  recognized  by  the  slope  of  the  rocks  in 
the  North  and  South  Downs,  which  are  the  fragments 
of  the  old  arch,  of  which  the  middle  part  has  been 
removed. 

27 


Development  of  Modern  Geology 

Other  mountain-chains,  such  as  the  Juras,  consist  of 
corrugated  layers  of  rock,  which  were  once  horizontal, 
and  have  clearly  been  raised  by  the  bending  of  the  crust. 
And  so  great  have  been  these  movements  that  on  the 
flanks  of  the  Himalaya,  rocks  formed  in  the  sea  have 
been  raised  20,000  feet  above  sea-level,  and  marine  rocks 
have  been  uplifted  at  least  12,000  feet  above  sea-level  in 
the  Alps. 

The  very  mobility  which  allows  the  sea  surface  to  be 
thrown  so  easily  into  waves  enables  the  sea  quickly  to 
recover  and  maintain  its  general  level.  That  water  will 
find  its  own  level  is  a  well-known  saying,  which  expresses 
an  important  truth.  The  sea  cannot  rise  in  one  place  much 
above  its  general  level,  and  the  mean  sea-level  is  taken 
as  the  standard  of  comparison  for  land-heights  through- 
out the  world.  Hence  geologists,  such  as  Playfair  in  1802, 
were  early  led  to  attribute  changes  in  the  relative  positions 
of  land  and  sea  to  movements  of  the  land. 

The  rise  and  fall  of  land  has  often  been  asserted  on 
quite  inadequate  grounds.  Thus  around  the  British  coasts 
there  are  many  "  submerged  forests,"  some  of  which  are  mis- 
named ;  they  are  often  regarded  as  proof  of  the  sinking  of 
the  land,  but  in  many  cases  these  beds  may  be  only  drifted 
vegetation,  which  was  carried  into  estuaries  by  floods  and 
there  sunk.  In  other  cases  these  layers  of  buried  wood 
are  truly  the  remains  of  old  land  surfaces ;  but  they  are 
now  below  sea-level  owing  to  the  shrinking  of  the  loose 
beds  of  silt  beneath  them,  and  this  shrinkage  by  the  drying 
of  waterlogged  beds  also  leads  to  the  sinking  of  old 
pavements  or  roads  below  sea-level. 

The  existence  of  a  fossil-plant  bed  below  sea-level  is 
therefore  no  certain  proof  of  a  sinking  of  the  land.  Con- 
vincing evidence  is,  however,  given  by  many  well-estab- 
lished cases  of  oscillation  of  the  coast-lands.  The  most 
famous  case  is  that  of  a  building  in  the  town  of  Pozzuoli, 

28 


Development  of  Modern  Geology 

on  the  shore  of  the  Mediterranean  to  the  west  of  Naples 
(Figs.  2  and  3).  Some  marble  columns  are  still  standing  ; 
their  lower  parts  were  buried  in  earth  until  they  were 
artificially  cleared  by  excavations  in  1750.  It  was  then 
found  that  the  marble  of  the  base  of  the  column  to  the 
height  of  twelve  feet  from  the  ground  retains  its  smooth 
surface,  but  for  the  next  nine  or  eleven  feet  the  marble 
is  rough,  and  has  been  excavated  by  sea-dwelling  animals, 
whose  shells  are  still  in  the  holes  which  they  bored.  The 
explanation  of  these  facts  has  led  to  one  of  the  most 
instructive  discussions  in  the  history  of  geology.  The 
columns  are  the  remains  of  a  building  which  is  generally 
known  as  the  Temple  of  Jupiter  Serapis,  but  it  is  said  to 
have  been  more  probably  a  public  bathing-hall.  It  has 
undergone  a  prolonged  oscillation  in  level.  It  was  built 
about  the  year  80  B.C.,  and  was  then  about  twelve  feet 
above  sea-level ;  the  ground  sank,  but  in  the  third  century 
A.D.  the  building  was  still  separated  from  the  sea  by  a 
road,  for  a  regulation  was  then  passed  as  to  the  repair 
of  the  road  between  it  and  the  sea.  Subsequently,  the 
land  sank  below  the  sea-level ;  the  exact  date  is  unknown, 
but  it  was  probably  at  the  eruption  of  an  adjacent  volcano, 
the  Solfatara,  in  1198.  The  movement  was  so  gentle  that 
all  the  columns  were  not  overthrown,  though  those  left 
standing  were  slightly  tilted  from  the  vertical ;  volcanic 
material  covered  the  bases  of  the  columns  up  to  the  height 
of  twelve  feet,  and  by  the  sinking  of  the  land  the  columns 
stood  in  the  quiet  waters  of  a  bay.  Sea  animals  then 
bored  into  the  upper  part  of  the  marble  columns  for 
a  height  of  nine  feet,  while  the  whole  surface  exposed  to 
the  water  was  roughened  by  the  roots  of  adhering  sea- 
weeds. Early  in  the  sixteenth  century,  and  probably  in 
1538,  at  the  eruption  which  formed  the  adjacent  volcano 
of  Monte  Nuovo  (the  New  Mountain),  the  locality  was 
quietly  uplifted  till  the  columns  were  again  raised  above 

29 


Development  of  Modern   Geology 

the  sea.  During  the  early  part  of  the  nineteenth  century 
the  level  sank  again  for  about  two  feet,  and  the  floor  of 
the  ancient  building  is  now  almost  exactly  at  sea-level. 

This  temple  affords,  therefore,  conclusive  evidence  of 
the  alternate  rising  and  sinking  of  the  land  through  a 
range  of  about  thirty  feet ;  and  the  movement  was  so 
quiet  that  some  of  the  columns  remained  standing,  and  are 
only  very  slightly  tilted  from  the  vertical.  This  striking 
instance  of  the  interchange  of  land  and  sea  was  at  first 
attributed  to  the  rise  and  fall  of  the  sea.  The  evidence  is, 
however,  conclusive  in  neighbouring  localities  that  the  sea 
and  land  have  remained  at  their  present  levels  for  the  past 
two  thousand  years,  and  these  variations  of  sea-level  could 
not  have  been  limited  to  that  one  locality  on  the  Bay  of 
Naples.  Hence  repeated  attempts  have  been  made  to  find 
some  explanation  that  does  not  include  the  rise  and  fall  of 
the  land.  The  poet  Goethe  suggested  that  the  site  of  the 
building  had  been  covered  by  a  lake ;  he  recognized  that 
the  shellfish  which  had  bored  into  the  columns  could  only 
have  lived  in  salt  water ;  but  he  thought  that  the  lake  was 
due  to  a  bank  piled  up  by  a  volcanic  eruption,  and  that 
the  water  had  been  rendered  so  salt  by  solution  from  the 
volcanic  materials  that  marine  animals  could  live  in  it. 
Prof.  Suess  of  Vienna  has  summarized  the  voluminous 
literature  on  this  famous  temple,  and  carefully  considered 
the  various  theories  in  regard  to  it.  But  he  found  no 
method  of  explaining  the  facts  except  by  an  oscillation  of 
the  land.  He  insists,  however,  that  the  uplift  was  only  a 
small  local  heaving  of  the  ground  consequent  on  the 
eruptions  of  the  adjacent  volcano,  and  that  it  does  not 
prove  the  possibility  of  the  uplift  of  large  areas. 

Prof.  Suess's  contention  that  the  movement  at  Pozzuoli 
was  small  both  in  area  and  height  is  no  doubt  just ;  but 
this  locality  is  a  geological  classic,  because  its  proof  that 
the  solid  land  can  steadily  rise  and  fall  struck  the  first 

30 


Development  of  Modern  Geology 

blow  at  the  old  doctrine  of  Catastrophes.  Henceforth  it 
was  unnecessary  to  invoke  universal  floods  to  explain  the 
evidence  of  sea-formed  rocks  on  dry  land.  If  such  floods 
had  happened,  they  would  have  destroyed  most  animal  life 
on  land,  while  the  great  disturbance  of  the  sea  might  have 
killed  off  many  marine  animals.  But  if  the  sea  spread  on 
to  the  land  in  consequence  of  local  uplifts  and  sinkings, 
then,  though  the  animals  of  one  area  might  be  destroyed, 
there  would  be  plenty  left  elsewhere  to  maintain  the  race. 
The  doctrines  taught  by  Lyell  have  been  generally 
adopted  as  the  fundamental  principles  of  geology.  The 
most  serious  attempt  to  question  them  has  been  made  by 
Prof.  E.  Suess.  Lyell  represented  the  world  as  having  a 
heaving  crust,  parts  rising  while  other  parts  sagged  down- 
ward. Suess,  on  the  other  hand,  represents  the  earth  as  a 
shrinking  globe.  In  places  the  rocks  are  upraised  in  folds, 
and  they  are  then  always  tilted  from  the  horizontal.  He 
maintains  that  it  is  a  physical  impossibility  for  wide 
areas  of  the  earth's  crust  to  be  upraised  without  being 
dislocated  and  shattered.  Widespread  even  movements, 
he  holds,  are  always  downward,  as  the  crust  sinks  when  its 
support  is  withdrawn  by  the  shrinkage  of  the  interior  of 
the  earth.  Accordingly  Prof.  Suess  holds  that  many  of  the 
facts  which  have  been  taken  to  prove  the  uprise  of  the 
land  are  due  to  the  sinking  of  the  sea.  He  holds  that 
the  level  of  the  sea  is  not  fixed  and  invariable.  There  can 
be  little  doubt  that  the  sea  surface  is  not  absolutely  level 
or  invariable,  for  many  factors  cause  local  irregularities  in 
sea-level.  Thus  the  wind  pushes  the  water  before  it.  A 
prevalent  wind  blowing  across  a  sea  will  blow  the  water 
away  from  the  one  shore  and  pile  it  up  against  a  lee  shore. 
An  estuary  widely  open  to  the  prevalent  wind  may  have 
the  level  higher  than  an  adjacent  arm  of  the  sea  with  a 
narrow  or  shallow  mouth ;  and  the  deposition  of  a  shoal 
across  an  estuary  may  permanently  lower  the  average 


Development  of  Modern  Geology 

level  of  the  water  within  it.  Two  adjacent  inland  seas 
may  therefore  have  slightly  different  water-levels ;  but  if 
the  channel  between  them  be  increased  in  width,  the  two 
seas  would  take  up  the  same  level. 

The  principle  that  water  finds  its  own  level  is  only  true 
of  water  which  is  throughout  of  the  same  composition  and 
weight.  Water  in  different  localities  varies  considerably 
in  saltness,  and  therefore  in  weight.  The  sea-water  is 
freshest  and  lightest  in  the  polar  regions,  and  in  such  seas 
as  the  Baltic,  which  receive  large  supplies  of  river-water. 
The  sea-water  is  saltest  and  heaviest  in  the  tropics,  as 
evaporation  is  greatest  there.  Where  the  sea  is  especially 
salt,  its  level  must  be  a  little  lower  than  in  areas  where  it 
is  less  salt.  There  are,  then,  several  factors  tending  to  keep 
the  sea-level  irregular,  and  any  change  in  these  factors 
would  tend  to  increase  or  reduce  these  irregularities. 
Hence  the  raised  beaches  along  the  coasts  of  Scotland 
and  Norway,  which  show  that  the  sea  once  stood  about 
twenty-five,  at  another  time  fifty,  and  at  another  one 
hundred  feet  above  the  present  level,  may  be  due  to  a 
variation  in  the  sea-level,  and  need  not  be  proof  of  an 
actual  uprise  of  the  land.  It  is  therefore  safer  to  say  that 
the  land  has  emerged  from  or  been  submerged  by  the  sea, 
instead  of  saying  that  the  land  has  risen  or  fallen ;  for  the 
emergence  of  the  land  may  be  due  to  the  movement  of 
either  land  or  sea. 

The  long  investigation  of  this  problem  leaves  little  doubt 
that  the  land  may  rise  as  well  as  fall,  and  in  all  probability 
the  Scottish  raised  beaches  mark  successive  halts  during 
the  uprise  of  the  land.  But  in  other  cases  Prof.  Suess's 
view  that  the  sea-level  has  altered  seems  the  most  probable 
conclusion. 

In  the  eastern  Alps,  in  Tyrol,  some  of  the  most  con- 
spicuous mountains  are  composed  of  limestones,  which 
were  formed  in  the  sea,  though  they  are  now  from  10,000  to 

32 


Development  of  Modern  Geology 

12,000  feet  above  sea-level.  In  the  Rocky  Mountains, 
also,  there  are  large  areas  of  rocks  that  were  laid  down  on 
the  sea-floor  and  are  now  situated  from  10,000  to  12,000 
feet  above  sea-level.  In  Tyrol  the  sheets  of  limestone  are 
bent  and  twisted,  and  are  often  so  tilted  that  they  stand  on 
end ;  and  these  disturbances  show  that  the  rocks  have 
been  flung  to  their  present  heights  during  the  earth  move- 
ments that  formed  the  Alps.  In  the  Rocky  Mountains,  on 
the  other  hand,  the  rocks  are  still  horizontal,  and  almost 
undisturbed ;  the  fractures  that  cross  the  rocks  are  due  to 
the  sinking  of  the  adjacent  land,  and  there  is  no  evidence 
of  any  uplift.  Hence  it  is  very  probable  that  when  these 


FIG.  2. — DIAGRAM  ILLUSTRATING  THE  RISE  OF  THE  SEA  UPON  THE 
LAND  IN  CONSEQUENCE  OF  THE  UPRISE  OF  THE  SEA-FLOOR. 

As  the  sea-floor  in  B  has  been  upraised  from  c  to  d,  the  displaced  water 
has  risen  from  e  to  /,  and  submerged  the  two  lower  terraces  on  the 
surrounding  lands. 

rocks  were  deposited,  the  earth  was  larger  than  at  present, 
so  that  the  sea-floor  was  from  10,000  to  12,000  feet  above 
the  present  sea  surface.  The  sea-level  has  fallen  by 
repeated  sinking  of  the  sea-floor;  and  these  horizontal 
marine  beds  in  the  Rocky  Mountains  have  been  left  where 
they  were  first  deposited,  and  are  a  monument  of  the 
former  level  of  the  sea. 

The  variation  in  level  of  the  sea  surface  is  also  indicated 
by  the  evidence  that  at  different  periods  in  the  earth's 
history  the  sea  has  advanced  upon  the  land  in  all  parts 
of  the  world.  These  movements  are  known  as  "  marine 
transgressions,"  as  the  sea  then  transgressed  or  trespassed 

33  c 


Development  of  Modern  Geology 

upon  the  land.  At  other  times  the  land  has  gained  upon 
the  sea  in  many  scattered  districts.  Such  variations  in 
the  sea-level  would  inevitably  result  from  widespread 
movements  of  the  sea-floor.  If  an  ocean  floor  sinks  so 
that  the  capacity  of  that  ocean  is  much  increased,  then 
the  sea-water  from  all  parts  of  the  world  would  flow  into 
that  ocean  and  the  sea-level  would  be  lowered  throughout 
the  world.  If  there  is  a  shallowing  of  the  great  oceans  by  an 
uprise  of  the  floor,  there  would  follow  a  world-wide  advance 
of  the  sea  upon  the  land,  or  a  great  marine  transgression. 
The  possibility  of  these  movements  was  recognized 
by  Lyell,  but  it  is  to  Prof.  Suess  that  we  owe  the 
recognition  of  their  great  importance  in  geological 
history.  They  afford  the  most  precise  available  method 
of  determining  the  dates  in  the  earth's  history  for 
widely  sundered  areas.  Moreover,  such  changes  in  sea- 
level  must  have  occasioned  climatic  and  geographical 
changes  of  world-wide  importance.  A  rise  in  sea-level 
would  have  increased  the  proportion  of  the  earth's 
surface  covered  by  water,  and  thus  have  affected  the 
climate  of  the  world ;  and  a  quick  change  in  climate  would 
have  stimulated  the  rapid  development  of  new  animals 
and  plants  and  led  to  the  extermination  of  those  which 
could  not  adapt  themselves  to  the  changing  conditions  of 
the  world  around. 


34 


CHAPTER  II 

THE  BIRTH  OF  THE  EARTH 

THE  earth  is  one  of  the  members  of  that  group  of  heavenly 
bodies  that  constitute  the  Solar  System,  of  which  the  sun 
is  the  central  and  largest  member.  And  our  solar  system 
is  itself  only  one  of  innumerable  similar  systems  which 
are  scattered  through  space.  The  members  of  the  other 
systems  we  see  at  night,  as  stars  spread  in  such  myriads 
over  the  sky;  and  as  these  stars  represent  worlds  in  all 
stages  of  development,  we  naturally  expect  from  astronomy 
the  most  direct  help  as  to  the  earliest  history  of  our  earth 
and  suggestions  as  to  its  future. 

Astronomy  has  discovered  that  the  sun  is  at  least  partly 
composed  of  gas  ;  whereas  other  heavenly  bodies,  like  the 
moon,  are  cold  and  solid.  The  sun  is  too  hot  and  the 
moon  too  cold  for  the  existence  of  our  higher  animals  and 
plants.  Bodies  like  the  sun  continually  give  forth  heat  ; 
and  unless  the  heat  be  renewed  they  must  become  colder  ; 
large  bodies  take  longer  to  cool  than  small  ones,  and 
therefore  if  the  sun,  earth,  and  moon  had  once  been  all  at 
the  same  temperature,  the  moon  would  have  cooled  down 
most  quickly.  It  would  have  passed  ages  ago  through 
the  stage  at  which  the  earth  is  now,  and  the  sun  would 
ultimately  reach  the  same  condition.  If  so,  the  moon  is  a 
dead  earth,  and  the  sun,  judged  by  the  standard  of  our 
earth,  is  still  immature. 

In  a  still  more  primitive  condition  the  sun  was  one  of 
those  heavenly  bodies  known  as  "  nebulae,"  which,  accord- 

35 


The   Birth  of  the  Earth 

ing  to  the  famous  nebular  theory1  *  of  the  great  French 
astronomer  Laplace,  represent  the  primitive  condition  of 
such  groups  of  stars  as  the  solar  system. 

According  to  that  theory,  "  this  world  was  once  a  fluid 
haze  of  light,"  for  it  and  all  the  other  members  of  the 
Solar  System  were  once  part  of  a  vast  cloud  of  very  hot 
gas,  which  extended  throughout  the  area  around  the  sun 
to  the  distance  of  the  most  remote  members  of  the  Solar 
System.  According  to  the  theory,  this  cloud  of  glowing 
gas  has  been  slowly  condensing,  and  as  it  condensed  it 
broke  up  into  smaller  clouds.  The  central  and  largest 
part  now  forms  the  sun,  and  being  by  far  the  largest  it  has 
longest  maintained  its  heat,  and  is  still  a  mass  of  incan- 
descent gas. 

The  nebula  was  assumed  to  be  spinning  around  its 
centre,  to  be  slowly  contracting  in  size,  and  to  be  cooling 
by  the  loss  of  the  heat  radiated  into  space.  As  the  nebula 
contracted,  it  was  supposed  occasionally  to  throw  off  its 
outer  ring ;  each  ring  then  condensed  into  a  planet,  and 
continued  to  move  around  the  centre  of  the  Solar  System 
in  the  same  direction  as  before.  Each  planet  was  thought 
in  turn  to  throw  off  smaller  rings  which  formed  its  moons. 
As  the  smaller  bodies  cooled,  they  became  first  molten,  and 
then  solid,  with  a  cold  crust  and  an  intensely  hot  interior. 

This  theory  was  consistent  with  so  many  of  the  chief 
facts  in  regard  to  the  Solar  System  that  it  soon  gained, 
and  appears  still  tp  hold,  general  acceptance.  It  explains 
why  all  the  planets  of  the  Solar  System  revolve  around 
the  sun  in  nearly  the  same  plane ;  for,  as  the  solar  nebula 
would  throw  off  its  rings  of  gas  at  its  equator,  the  planets 
formed  from  them  would  continue  to  move  along  paths 
lying  near  the  equatorial  plane  of  the  nebula.  Moreover, 
the  members  of  the  Solar  System,  with  a  few  insignificant 
exceptions,  revolve  around  the  sun  in  the  same  direction 
*  For  references  see  end  of  chapters. 

36 


The  Birth  of  the  Earth 

as  if  they  still  retained  the  movement  of  the  vast  mass  in 
which  they  were  once  included.  It  has  been  calculated 
that  the  odds  against  the  660  members  of  the  Solar 
System  revolving  in  the  same  direction  as  the  result  of 
chance,  would  be  many  billions  of  trillions  to  one. 

The  theory,  moreover,  was  consistent  with  the  views  as 
to  the  constitution  of  nebulae  long  current  among  astrono- 
mers. The  revelations  by  Lord  Rosse's  great  telescope 
for  a  time  threw  doubt  upon  the  theory,  for  it  proved  that 
many  of  the  nebulae  are  only  clusters  of  stars.  Hence 
most  astronomers  expected,  and  some  still  expect,  that  all 
the  rest  would  prove  to  be  the  same.  The  spectroscope, 
however,  reversed  the  trend  of  opinion  started  by  the 
telescope,  for  with  it  Sir  William  Huggins  showed  that 
though  some  of  the  nebulae  may  be  solid,  or  may  consist 
of  clusters  of  stars  like  the  sun,  others  appear  to  consist 
of  incandescent  gas.  He  also  claimed  that  in  addition  to 
pseudo-nebulae,  or  star  clusters,  there  are  true  nebulae 
composed  of  incandescent  gas.  These  true  nebulae  have 
therefore  the  constitution  required  by  Laplace's  theory. 

Moreover,  the  examination  of  the  nebulae  by  the  aid  of 
more  powerful  telescopes,  aided  by  photography,  revealed 
many  new  facts  that  were  consistent  with  Laplace's  theory. 
Thus  the  great  nebula  in  Andromeda  was  found  to  be  a 
disc  with  a  large  central  glowing  mass  and  a  fainter  less 
luminous  envelope ;  and  this  outer  envelope  consists  of  a 
series  of  irregular  rings  containing  lesser  patches  or  knots, 
that  might  be  regarded  as  embryonic  planets.  Dr.  Robert's 
photograph  of  the  spiral  nebula  in  the  Constellation  of 
the  Hunting  Dogs  shows  the  same  feature  ;  and  this  nebula 
appears  circular  instead  of  a  thin  oval,  since  from  the 
earth  we  look  straight  down  upon  the  disc,  whereas  we 
see  the  great  nebula  of  Andromeda  obliquely  ;  while  other 
nebulae  appear  like  straight,  narrow  bars,  with  rounded 
ends,  as  we  only  see  their  edges.2  Another  fact  that  was 

37 


The  Birth  of  the  Earth 

regarded  as  consistent  with  Laplace's  theory  was  that 
some  nebulae  appear  to  be  ring-shaped.  The  typical 
example  is  in  the  constellation  Lyra,  and  has  been  known 
since  1779.  The  use  of  better  telescopes  led  to  the  dis- 
covery of  other  ring-shaped  nebulae,  though  it  is  now 
believed  by  some  astronomers  that  these  ring-shaped 
nebulae  are  really  spiral,  and  only  appear  ringed  from  the 
angle  at  which  they  are  seen.  Laplace's  theory  was  also 
confirmed  in  popular  favour  by  its  agreement  with  the 
view  of  Helmholtz  that  the  heat  of  the  sun  is  maintained 
by  contraction  of  its  mass.  Owing  to  the  many  attractive 
features  in  the  nebular  theory,  it  was  at  one  time  univer- 
sally accepted;  but  so  many  important  facts  have  been 
discovered  which  are  opposed  to  it  that  See  has  recently 
declared  that  the  theory  is  already  "  thrice  slain,"  and 
that  there  is  "no  course  open  to  us  but  to  personally 
and  unconditionally  abandon  it."3  Thus  the  satellites  of 
Uranus  and  Neptune,  and  the  more  recently  discovered 
satellite  of  Saturn,  revolve  in  the  wrong  direction,  It  is 
true  Sir  Robert  Ball  has  pointed4  out  that  the  satellites 
of  Uranus  that  revolve  backwards  move  in  a  plane  which 
is  at  the  angle  of  83°  to  the  orbit  of  that  planet ;  the  direc- 
tion of  their  movement  may  be  explained  by  their  orbits 
having  been  tilted  through  more  than  a  right  angle,  so 
that  the  direction  in  which  these  moons  revolve  around 
Uranus  may  have  been  reversed,  just  as  when  a  watch  is 
tilted  so  that  it  begins  to  face  downward  the  direction  of 
rotation  of  the  hands  is  reversed.  The  orbit  of  the  satellite 
of  Neptune  is  inclined  at  the  angle  of  only  35°  to  that  of 
the  planet ;  so  that  the  reversal  of  the  movement  of  this 
satellite  would  require  the  tilting  of  its  orbit  through  about 
145°-  The  exceptions  are  so  few  in  comparison  with  the  660 
members  of  the  Solar  System  which  obey  the  rule  that 
most  authorities  do  not  consider  that  they  seriously  weaken 
Laplace's  theory. 

38 


THE  GREAT  NEBULA  IN  THE  CONSTELLATION  OF  ORION 

Showing  its  irregular  shape  and  tenuity,  as  even  minor  stars  are  seen  clearly  through  it. 


The  Birth  of  the  Earth 

Another  difficulty  is  that  the  rotations  of  the  inner 
satellite  of  Mars  and  of  the  inner  part  of  the  ring  of 
Saturn  are  more  rapid  in  reference  to  the  rate  of  the 
adjacent  planets  than  can  be  explained  by  the  nebular 
theory.  But  these  inconsistent  velocities  are  so  few  and 
occur  in  such  relatively  insignificant  bodies  that  the 
exceptions  hardly  disprove  the  rule. 

Far  more  weighty  are  two  mathematical  objections  due 
to  the  late  Prof.  Moulton  of  Chicago.  He  calculated 
that  if  the  matter  of  the  Solar  System  were  spread  out  as 
a  nebula  throughout  the  whole  space  occupied  by  the 
Solar  System  (i.e.,  within  the  orbit  of  Neptune),  its  rota- 
tion would  not  cause  the  outer  zones  to  break  away  from 
the  rest  of  the  nebula.  His  second  objection  is  that  the 
outer  planets  move  around  the  sun  at  too  high  a  speed  for 
their  energy  to  have  been  inherited  from  a  gaseous  nebula. 
Thus  Jupiter  and  its  satellites  include  less  than  one- 
thousandth  of  the  matter  in  the  Solar  System ;  but  they 
have  95  per  cent,  of  the  total  energy  of  rotation.  Moulton 
therefore  concluded  that  the  distribution  of  energy  in 
the  Solar  System  is  fatal  to  the  truth  of  the  nebular 
theory. 

The  greatest  improbability  in  Laplace's  theory  concerns 
its  heat-supply.  The  cold  of  outer  space  is  so  intense  that 
a  mass  of  diffuse  gas  could  not  long  maintain  its  heat.  A 
nebula  contains  a  very  small  amount  of  matter  in  propor- 
tion to  the  area  it  occupies,  and  a  cloud  of  gas  or  dust 
would  not  remain  incandescent  for  a  long  period  unless 
its  heat  were  constantly  renewed.  Thus,  according  to 
Prof.  See,  "  the  feebleness  of  the  light  of  the  nebulae  and 
their  obvious  transparency  corresponds  to  luminescence 
in  space,  and  enables  us  to  see  that  on  the  average  they 
are  much  rarer  than  the  vacuum  of  an  air-pump.  Such 
a  mass  must  be  at  a  low  temperature,  but  little  above  the 
absolute  zero  of  space.  If  such  a  tenuous  swarm  were 

39 


The  Birth  of  the  Earth 

heated,  the  heat  would  be  radiated  away  in  an  instant, 
owing  to  the  great  transparency  of  the  cloud." 

This  difficulty  was  overcome  by  the  meteoritic  hypothesis 
of  Sir  Norman  Lockyer.6  According  to  that  theory  a  nebula 
instead  of  being  a  cloud  of  gas  is  a  swarm  of  solid  meteorites. 
The  meteorites  are  popularly  known  as  "  shooting-stars," 
"  fire-balls,"  and  "  thunderbolts."  On  any  clear  cloudless 
night  it  is  only  necessary  to  watch  the  sky  for  a  short  time  to 
see  a  shooting-star  flash  like  a  rocket  across  the  sky.  These 
shooting-stars  are  meteorites  which  have  entered  the  earth's 
atmosphere,  and  being  heated  by  the  friction  as  they  rush 
through  it,  their  outer  skin  is  rendered  white  hot.  Some- 
times the  shooting-stars  are  seen  to  leave  a  luminous  trail 
like  hot  sparks  behind  them,  which  is  due  to  the  white-hot 
ragments  torn  off  them. 

These  meteorites  are  so  abundant  that  it  is  estimated 
that  any  observer  may  see,  on  an  average,  from  eight  to 
ten  every  hour  on  any  clear  starlight  night.  Most  of  them 
are  small,  but  others  must  be  of  considerable  size ;  in  1783 
a  great  meteorite  crossed  Europe  from  the  North  Sea  to 
the  Mediterranean  at  the  speed  of  thirty  miles  a  second, 
and  gave  off  as  much  light  as  the  full  moon.  That  these 
meteorites  were  solid  bodies  which  often  fell  upon  the 
earth  was  for  long  dismissed  as  incredible.  It  is  true 
that  one,  270  pounds  in  weight,  was  known  to  have  fallen  at 
Ensisheim  in  Elsass  in  1492.  But  it  was  not  generally 
believed  until  1809  that  the  materials  known  as  "aerolites," 
or  "  skystones,"  were  really  meteorites,  that  had  fallen  from 
the  sky;  and  it  is  now  generally  considered  that  many 
famous  stones  are  meteorites,  such  as  the  Kaaba,  the 
sacred  stone  in  the  Mosque  at  Mecca,  and  on  a  meteorite 
was  based  the  tradition  of  the  shield  of  Mars  kept  by  the 
Salii  in  ancient  Rome. 

According  to   Sir  Norman  Lockyer,  nebulae   are  vast 
collections  of  such  meteorites,  and  the  light  of  the  nebula 

40 


The   Birth  of  the  Earth 

is  maintained  by  the  heat  due  to  the  constant  collisions 
between  the  separate  meteorites.  The  members  of  a  swarm 
of  meteorites  would  be  banged  together  like  stones  in  a  bag 
that  is  being  hurled  around  in  the  air,  and  the  heat  due 
to  these  collisions  would  convert  parts  of  the  meteorites 
into  incandescent  vapour. 

Sir  Norman  Lockyer's  theory  represents  comets  and 
stars,  as  well  as  nebula,  as  swarms  of  meteorites  enveloped 
in  a  cloud  of  meteoritic  vapour  due  to  the  heat  generated 
by  constant  collisions.  Nebulae  resemble  comets  in  appear- 
ance owing  to  their  hazy  light  and  transparency,  and  there 
is  much  evidence  to  show  that  comets  are  composed  of  the 
same  materials  as  meteorites.  A  meteorite  which  fell  in 
Tyrol  in  May,  1910,  has  been  identified,  though  perhaps 
on  inadequate  grounds,  as  a  fragment  of  Halley's  Comet ; 
and  the  evidence  seems  conclusive  that  several  comets 
have  been  altered  into  swarms  of  meteorites.  Thus  Biela's 
Comet,  which  reappeared  at  intervals  of  6  years  244  days 
from  1772  to  1852,  was  found  in  1845  to  have  divided  into 
two.  On  its  next  appearance  in  1852  the  halves  were 
widely  separated.  It  was  never  seen  again ;  but  its  place 
was  taken  by  a  swarm  of  shooting-stars,  which  were 
especially  brilliant  in  1872  and  1885.  The  Aquarid 
meteors  of  May,  1910,  may  in  like  manner  be  part  of 
Halley's  Comet,  and  an  iron  meteorite  referred  to  this 
swarm  fell  in  Mexico  in  1885. 

According  to  Sir  Norman  Lockyer's  theory,  some  parts 
of  space  were  once  densely  crowded  with  meteorites,  which 
were  collected  into  swarms  and  thus  formed  nebulae.  The 
central  parts  of  the  swarm  would  be  intensely  heated  by 
contraction,  and  the  separate  meteorites  in  a  swarm  would 
gradually  collect  into  a  smaller  space  and  then  become 
closely  packed  together,  and  be  finally  welded  into  one 
compact  mass  and  form  a  star.  Meanwhile  the  constant 
collisions  between  the  outer  meteorites,  as  they  travelled 

4* 


The   Birth  of  the  Earth 

around  the  central  body,  would  maintain  an  envelope  of 
luminous  incandescent  meteoritic  gas.  The  meteorites 
would  be  slowly  collected  into  dense  masses,  which  would 
be  welded  by  heat  into  compact  bodies,  and  thus  would 
give  rise  to  the  separate  members  of  the  various  stellar 
systems. 

According,  then,  to  this  theory,  the  different  stars  and 
star  systems  have  been  formed  from  swarms  of  meteorites. 
Such  a  swarm  of  meteorites  would  behave  like  a  gas,  as 
shown  by  the  late  Sir  George  Darwin,  each  meteorite 
representing  a  particle  of  the  gas.  Hence  the  chief  argu- 
ments used  to  support  Laplace's  theory  would  apply  also 
to  the  meteoritic  theory,  which  is  free  from  the  physical 
difficulty  of  the  maintenance  of  the  heat. 

The  chief  obstacle  to  the  meteoritic  theory  when  pro- 
pounded by  Sir  Norman  Lockyer  was  the  spectroscopic 
evidence.  The  spectroscope  consists  essentially  of  a 
triangular  prism  of  glass.  A  point  of  ordinary  light 
looked  at  through  a  prism,  such  as  the  lustre  of  an  old- 
fashioned  chandelier,  appears  like  a  band  of  coloured 
light,  ranging  from  violet  on  one  side  to  red  on  the  other. 
Such  a  band  of  light  is  known  as  a  "  spectrum,"  and  the 
spectra  given  by  white-hot  solid  bodies,  or  from  luminous 
liquids  and  very  dense  gases,  is  a  continuous  band  of  light 
ranging  from  violet  to  red.  A  second  kind  of  spectrum 
consists  of  a  band  of  light  broken  by  many  dark  bars  or 
"  absorption-lines,"  and  such"  dark-line  spectra"  are  due 
to  light  passing  through  a  medium  which  absorbs  some 
of  the  rays  of  light.  Different  materials  absorb  different 
rays,  and  thus  produce  different  series  of  dark  lines  or 
bands.  Hence  by  mapping  these  dark  bars  the  chemical 
composition  of  the  material  traversed  by  the  light  can 
be  determined.  The  composition  of  the  outer  zone  of 
the  sun  has  been  discovered  from  the  dark  bars  in  its 
spectrum. 

42 


The  Birth  of  the  Earth 

Spectra  of  the  third  kind  consist  of  bright  lines.  Such 
spectra  are  given  by  incandescent  gas ;  and  the  bright 
lines  given  by  an  element  occupy  the  same  positions  as  its 
absorption-lines  in  a  dark-line  spectrum. 

It  was  discovered  by  Sir  William  Huggins  in  1862  that 
the  Great  Nebula  of  Orion  has  a  bright  line  spectrum.  It 
was  therefore  considered  that  this  nebula  was  composed 
of  incandescent  gas,  and  its  lines  are  interpreted  as  show- 
ing that  it  consists  of  hydrogen,  helium,  and  the  element 
nebulium,  which  is  known  only  in  nebulae. 

The  spiral  nebula  in  Andromeda  was  shown  by  Scheiner 
to  give  a  faint  continuous  spectrum  traversed  by  dark 
lines.  This  nebula  has  therefore  been  regarded  as  com- 
posed of  a  central  mass  of  solid  incandescent  material, 
which  is  surrounded  by  a  layer  of  gas.  It  is  therefore, 
according  to  the  spectroscopic  evidence,  similar  in  structure 
to  the  sun. 

Accordingly  it  has  been  confidently  claimed  that  those 
nebulae  which  have  bright-line  spectra  are  gaseous,  and 
that  those  with  dark-line  spectra  consist  of  solid  particles  ; 
and  this  conclusion  is  consistent  with  Laplace's  theory. 
Nevertheless,  further  research  has  thrown  doubt  on  the 
simple  interpretation  of  the  spectroscopic  evidence  regard- 
ing the  structure  of  nebulas.6  It  is  probable  that  many  of 
the  nebulae  are  composed  of  solid  materials  surrounded 
or  accompanied  by  layers  of  meteoritic  gas  or  dust. 

This  is  the  structure  required  by  the  meteoritic  theory  ; 
but  the  spectroscope  has  been  considered  to  throw  doubt 
on  that  theory,  as  it  has  not  proved  that  meteorites  and 
comets  are  composed  of  the  same  materials. 

The  meteorites  contain  many  of  the  elements  that  are 
common  in  the  earth's  crust.  Thus  Mingaye7  found  in 
the  Mount  Dyrring  meteorite  which  fell  in  New  South 
Wales  the  following  elements  : — aluminium,  calcium, 
carbon,  chlorine,  chromium,  cobalt,  copper,  gold, 

43 


The  Birth  of  the  Earth 

hydrogen,  iridium,  iron,  magnesium,  manganese,  nickel, 
oxygen,  palladium,  phosphorus,  platinum,  potassium, 
silicon,  sodium,  sulphur,  and  titanium.  There  can  be 
little  doubt  that  comets  consist  of  the  same  elements 
as  meteorites ;  but  that  view  has  not  been  learnt  from  the 
spectroscope ;  for  apart  from  the  reflected  sunlight,  the 
spectra  of  the  comets  only  show  the  lines  which  indicate 
the  presence  of  hydrocarbons  and  sodium,  but  not  those  of 
the  characteristic  meteoritic  metals. 

As  the  spectroscope  does  not  show  the  presence  of  the 
characteristic  meteoritic  elements  in  comets,  we  cannot 
trust  its  similar  negative  testimony  that  nebulas,  owing  to 
their  simple  spectra,  differ  in  ultimate  composition  from 
meteorites. 

This  meteoritic  hypothesis  has  been  further  developed 
by  Prof.  T.  C.  Chamberlin  of  Chicago.  He  maintains 
that  the  old  form  of  the  meteoritic  theory  is  inadequate, 
since  there  is  no  known  force  which  would  collect  the 
meteorites  that  are  scattered  irregularly  through  space. 
He  holds  that  the  force  of  gravity  would  be  inadequate  to 
bring  together  bodies  travelling  with  the  high  speed  attri- 
buted to  meteorites  unless  their  paths  brought  them  quite 
close  to  one  another.  He  therefore  thought  that  indepen- 
dent scattered  meteorites  would  never  be  collected  into 
dense  masses.  Many  meteorites,  however,  travel  in  regular 
orbits  around  the  sun,  and  these  would  sometimes  pass  on 
their  regular  paths  so  near  to  others  that  the  attraction 
of  gravity  would  be  sufficient  to  bring  them  together. 
The  meteorites  that  revolve  in  regular  orbits  around  the 
sun  may  be  regarded  as  minute  planets.  Prof.  Chamber- 
lin therefore  calls  them  "  planetesimals,"  an  abbreviation 
of  infinitesimal  planets ;  and  according  to  this  view  only 
meteorites  of  the  planetesimal  variety  are  concerned  in  the 
formation  of  nebulae. 

The  exclusion  of  all  other  meteorites  from  contributing 

44 


The  Birth  of  the  Earth 

to  nebulae  does  not  rest  on  an  altogether  satisfactory  basis  ; 
for  it  is  doubtful  whether  meteorites  travel  with  the 
enormous  velocities  sometimes  attributed  to  them.  This 
velocity  is  possibly  exaggerated;  for,  a$  Pickering8  has 
shown,  the  slight  penetration  into  the  ground  of  the 
meteorite  weighing  thirty-eight  tons  at  Cape  York,  Green- 
land, of  the  twenty-ton  Bacubirita  meteorite  of  Mexico, 
and  of  the  sixteen-ton  Williamette  meteorite  of  Oregon 
shows  that  each  was  moving  slowly.  A  cannon-ball  after 
passing  five  miles  through  the  air  has  overcome  as  much 
atmospheric  resistance  as  a  meteorite  that  has  fallen 
vertically  on  to  the  earth ;  yet  modern  projectiles,  after 
travelling  this  distance,  are  moving  with  a  much  higher 
speed  than  the  largest  meteorites  which  have  reached  the 
earth,  with  the  exception,  perhaps,  of  the  problematical 
meteorites  of  the  Devil's  Canyon  (see  pp.  138-140). 

The  second  objection  raised  to  the  original  meteoritic 
theory  is  that  the  supply  of  meteorites  is  inadequate  for 
the  formation  of  meteoritic  nebulae.  Prof.  Chamberlin 
holds  that  the  amount  of  meteoritic  material  in  space 
is  very  small.  There  does  not,  indeed,  seem  to  be  much 
matter  in  the  space  between  the  stars,  for  their  light 
is  obscured  at  night  by  the  intervention  of  a  mere  bucket- 
ful of  water  whipped  up  into  a  streak  of  cloud.  The  fact 
that  so  little  extra  matter  in  the  line  of  sight  between  us 
and  the  stars  makes  so  great  a  difference  suggests  that 
there  is  really  very  little  material  in  interstellar  space. 
Prof.  Chamberlin  holds  that  the  amount  of  meteoritic 
material  which  falls  on  the  earth  is  so  small  that  its 
accumulation  would  require  "  a  billion*  years  "  to  form  a 
layer  an  inch  in  thickness.  These  estimates  are  opposed 
to  those  of  many  other  authorities,  who  represent 

*  A  billion,  according  to  American  usage,  is  one  thousand  million  ; 
not  a  million  million,  as  in  English  usage.  The  latter  is  more  correct 
etymologically,  and  more  logical. 

45 


The  Birth  of  the  Earth 

meteorites  as  inconceivably  numerous,  as  no  less  than 
twenty  million  enter  the  earth's  atmosphere  every  day. 
During  the  great  display  of  1833  Arago  estimated  that 
240,000  meteorites  were  visible  above  the  horizon  at 
Boston  on  the  morning  of  November  13.  Some  of  these 
crowded  swarms  of  meteorites  are  very  long.  Thus  the 
earth,  between  November  13,  1865,  and  September,  1866, 
passed  through  one  vast  continuous  stream  of  meteorites 
500  million  miles  long ;  and  so  many  meteorites  are  seen 
by  the  telescope  which  are  invisible  to  the  naked  eye, 
that  Sir  Norman  Lockyer  estimates  the  number  which 
enter  the  earth's  atmosphere  at  about  400  million  every 
day. 

According  to  Prof.  H.  A.  Newton  each  part  of  space 
along  the  earth's  orbit  equal  in  size  to  the  earth  contains 
30,000  meteorites,  which  are  therefore  about  250  miles 
apart.  In  other  cases  they  are  far  more  crowded.  Thus 
in  the  Biela  swarm  they  were  only  twenty  miles  apart.  In 
the  November  display  of  1833  the  average  distance  has 
been  estimated  at  fifteen  miles,  and  in  that  of  1872  at  the 
distance  of  thirty-five  miles. 

The  estimates  of  the  amount  of  meteoritic  material 
added  yearly  to  the  earth  vary  greatly.  According  to 
Prof.  Chamberlin  it  is  "exceedingly  small  "9;  Arrhenius, 
on  the  contrary,  has  estimated  the  amount  at  20,000  tons 
a  year,  and  Prof.  Schwarz  advocates  the  view  that  the 
earth  has  grown  from  a  comparatively  small  body  to  its 
present  size  by  the  rain  of  meteorites  upon  its  surface. 

According  to  the  previous  estimates,  the  meteorites  are 
still  very  abundant  even  along  the  earth's  orbit ;  and  if  the 
planets  have  been  formed  from  meteorites,  the  areas  they 
traverse  were  probably  swept  comparatively  clear  during 
the  collection  of  the  meteoritic  material  into  planets- 
Sir  Norman  Lockyer  considers  that  the  area  of  the  Solar 
System  before  the  condensation  of  the  different  members 


The  Birth  of  the  Earth 

was  so  full  of  jostling  meteorites  that  he  calls  it  "  a 
meteoritic  plenum"  (Lat., plenus,  full  or  filled). 

The  origin  of  star  systems  from  cold-scattered  meteoritic 
material  has  also  been  advocated  by  Prof.  T.  J.  J.  See  of 
California ;  according  to  him  the  whole  of  space  is  rilled 
with  a  fine,  cosmic  meteoritic  dust.  Some  of  this  dust 
collects  and  condenses  into  stars,  while  its  aggregation 
also  forms  nebulas.  The  nebulae,  he  argues,  consist  of 
a  mixture  of  gas,  of  fine-grained  cosmic  dust,  of  meteorites, 
and  of  solid  globes  as  large  as  planets,  which  have  fallen 
into  the  nebula.  According  to  this  explanation,  the 
planets  and  satellites  of  the  Solar  System  are  bodies 
which  have  been  captured  by  it,  and  are  not  all  condensed 
fragments  of  one  original  nebula. 

Some  of  the  astronomers  who  have  in  recent  years  care- 
fully discussed  the  nebular  theory  are  emphatic  that  it 
is  quite  untenable  in  its  original  form.  Prof.  See  indeed 
remarks  that  "  probably  no  one  hereafter  will  ever  again 
give  serious  consideration  to  a  theory  which  is  shown  to 
be  absolutely  untenable." 

The  decision  between  the  three  rival  forms  of  the 
nebular  theory  is  a  question  for  the  astronomer  to  settle. 
The  geologist  is  prepared  to  accept  his  verdict  whether 
the  nebula  was  originally  composed  of  incandescent  gas, 
of  solid  meteorites,  or  of  See's  mixture.  The  main  con- 
cern of  the  geologist  is  whether  the  earth  began  as  a  mass 
of  hot  gas  or  of  cold  solid  material,  and  the  geologist  will 
prefer  to  judge  these  alternative  views  by  their  agreement 
with  the  geological  history  of  the  earth.  The  geological 
evidence  appears  to  agree  better  with  the  origin  of  the 
earth  from  a  nebula  of  solid  meteoritic  matter  than  from 
a  nebula  of  white-hot  gas. 

If  the  earth  began  as  incandescent  gas,  we  should  expect 
its  climate  to  show  a  slow  progressive  cooling.  There  is, 

47 


The  Birth  of  the  Earth 

however,  no  evidence  that  the  earth  has  undergone  any 
such  steady  refrigeration.  The  Archean  rocks  show  traces 
of  glacial  action  in  several  localities  and  horizons,  but  as 
they  are  in  comparatively  high  latitudes,  they  may  imply 
only  a  climate  as  cold  as  at  present.  In  the  middle  part 
of  the  Cambrian  Period,  which  is  the  earliest  that  con- 
tains well-preserved  traces  of  life,  there  are  glacial  de- 
posits which  prove  the  existence  of  ice  at  sea-level  in 
China  and  within  a  short  distance  of  the  tropics  in  South 
Australia.  Hence,  almost  at  the  beginning  of  the  geo- 
logical record,  some  districts  on  the  earth  had  a  colder 
climate  than  they  have  now.  There  have  been  many  local 
oscillations  of  temperature  through  the  world's  history,  but 
it  appears  that  the  mean  temperature  of  the  earth  has  not 
been  seriously  different  from  the  present  throughout  the  pro- 
longed period  of  which  the  geologist  has  definite  records. 

It  is  true  that  on  the  meteoritic  theory  the  earth  must 
once  have  had  a  higher  surface  temperature,  but  the  crust 
may  have  passed  through  its  hot  stage  very  quickly,  and 
have  soon  cooled  down  to  its  present  average  temperature. 
The  geological  history  of  climate  is  therefore  more  con- 
sistent with  the  meteoritic  than  with  the  gaseous  nebular 
theory. 

Again,  on  the  theory  of  the  incandescent  nebula  we 
should  expect  a  slow  thickening  of  the  hard  crust  of  the 
earth  and  a  steady  decline  in  the  intensity  of  volcanic 
action.  It  has  been  thought  that  volcanic  action  was 
originally  distributed  almost  universally  over  the  earth, 
that  at  some  early  periods  vast  floods  of  lava  were  dis- 
charged from  long,  broad  fissures  through  the  crust, 
that  modern  volcanoes  are  comparatively  insignificant  in 
number  and  power,  and  that  they  represent  only  a  dwind- 
ling stage  of  volcanic  activity.  If  this  conception  of  vol- 
canic history  had  been  confirmed,  it  would  have  supported 
Laplace's  theory.  We  find,  on  the  contrary,  that  through- 


The  Birth  of  the  Earth 

out  geological  history  periods  of  widespread  volcanic 
action  have  alternated  with  intervals  of  comparative 
volcanic  repose.  There  is  no  evidence  of  any  steady 
reduction  in  volcanic  intensity  such  as  would  be  expected 
if  the  earth  had  cooled  from  a  gaseous  nebula. 

Again,  we  might  expect,  if  the  earth  has  descended 
directly  from  a  gaseous  nebula,  to  find  traces  of  a  gradual 
but  important  change  in  the  composition  of  the  earth's 
atmosphere.  The  primeval  atmosphere  was  probably 
richer  in  carbon  dioxide  and  perhaps  poorer  in  oxygen 
than  the  present,  and  contained  various  gases  now  absent 
or  present  only  in  minute  traces ;  but  this  stage  was  ap- 
parently soon  passed,  and  the  atmosphere  settled  down  to 
a  composition  which  appears  to  have  remained  fairly  con- 
stant throughout  geological  times. 

The  validity  of  the  geological  arguments  as  to  the 
earth^s  origin  may  be  questioned,  since  it  may  be  said 
that  only  the  very  oldest  rocks  can  give  any  evidence  as 
to  the  relative  merits  of  the  different  nebular  theories; 
and  that  these  most  ancient  rocks  have  been  so  alt-red 
that  they  tell  us  nothing  as  to  the  geographical  conditions 
of  the  earth  when  they  were  being  made.  Below  the 
oldest  known  sedimentary  rocks  there  is  a  thick  crystalline 
zone,  which  may  include  the  material  deposited  while  the 
earth  was  cooling  down  to  its  present  temperature.  The 
lower  crystalline  layer  of  the  earth's  crust  may  represent 
a  period  compared  with  which  the  hundreds  or  thousands 
of  millions  of  years  of  geological  time  are  relatively  short ; 
and  any  traces  of  the  geographical  conditions  of  the  earth 
while  it  was  cooling  to  its  present  surface  temperatures 
may  have  been  buried  in  these  pre-Archean  rocks,  and 
then  destroyed  for  ever  when  they  were  altered  to  their 
present  states.  Geological  evidence,  however,  shows  us 
that  the  physical  and  geographical  agencies  on  the  earth 
have  been  of  approximately  the  same  strength  as  they  are 

49  D 


The  Birth  of  the  Earth 

at  present  throughout  the  whole  length  of  time  with  which 
it  deals  ;  and  this  conclusion  agrees  better  with  the  origin 
of  the  earth  from  a  swarm  of  solid  meteorites  or  meteoritic 
dust  than  from  a  cloud  of  incandescent  gas. 

1  The  theory  was  published  in  Laplace's  "  Systeme  du  Monde," 
1796,  and  was  more  fully  developed  in  the  edition  of  1824.    Primitive 
forms  of  the  theory  had  been  suggested  by  earlier  thinkers ;  the  best 
known  of  these  older  suggestions  is  that  by  the  philosopher  Kant. 

2  T.  J.  J.  See,  "  Researches  on  the  Evolution  of  the  Stellar  Systems," 
IQIO,  vol.  ii.,  Plate  A,  p.  548. 

8  Ibid.,  1910,  vol.  ii.,  p.  361. 

4  R.  S.  Ball,  "  The  Earth's  Beginning,"  1901,  p.  338. 

6  J.  N.  Lockyer,  "  The  Meteoritic  Hypothesis  :  A  Statement  of  the 
Results  of  a  Spectroscopic  Inquiry  into  the  Origin  of  Cosmical 
Systems"  (London,  1890). 

c  Continuous  spectra  may  be  caused  by  dense  gases  as  well  as  by 
solids  ;  a  dark-line  spectrum  may  appear  discontinuous  owing  to  the 
combination  of  a  continuous  and  a  bright-line  spectrum,  if  the  former 
be  the  brighter.  Further  uncertainty  has  been  introduced  by  the 
discovery  that  the  lines  or  bands  absorbed  or  given  off  by  a  material 
vary  with  its  physical  condition.  Thus  the  late  Agnes  Clerke  ("  The 
System  of  Stars,"  second  edition,  1905,  p.  66)  remarked  concerning 
some  nebulas  that  "  spectroscopically  (whatever  they  may  be 
physically)  they  are  mere  spheres  of  glimmering  gas,"  and  this 
statement  implies  that  the  spectroscope  evidence  is  no  certain  test 
of  the  real  physical  condition  of  a  material. 

Comte  de  Pluvinel  has  recently  asserted  the  same  doubt  respecting 
the  spectroscopic  evidence,  as  to  the  structure  of  comets,  and  he 
states  that  the  simple  conclusions  as  to  their  structure  that  were 
accepted  a  dozen  years  ago  are  now  rendered  uncertain  by  many 
fresh  discoveries  (vide  Nature,  February  15,  1912,  vol.  Ixxxviii., 
p.  526). 

E.  J.  Stone  (Proc.  Roy.  Soc.,  1877,  vol.  xxvi.,  pp.  156,  157,  517-519), 
in  1877,  advocated  the  view  that  even  the  nebulae  with  bright-line 
spectra  need  not  be  gaseous,  as  they  might  be  distant  clusters  of 
solid  stars  surrounded  by  a  gaseous  envelope.  And  the  view  that 
nebulae  are  star  clusters  has  been  reaffirmed  by  Dr.  Fath  of  the  Lick 
Observatory,  who  denies  that  anyjnebulae  have  continuous  spectra 
(Lick  Observat.,  Bulletin  No.  149  ;  vide  also  See,  op.  cit.,  p.  556). 

Although  therefore  the  spectroscopic  evidence  may  leave  it 
doubtful  whether  the  nebulae  are  gaseous  or  solid,  it  is  at  least 

50 


The  Birth  of  the  Earth 

consistent  with  the  view  that  many  of  them  are  composed  of  solid 
materials  as  is  required  by  the  meteoritic  theory. 

T  J.  C.  H.  Mingaye,  "  Notes  on  and  Analyses  of  the  Mount  Dyrring, 
Barraba,  and  Cowra  Meteorites"  ("  Records  of  the  Geological  Survey 
of  New  South  Wales,"  1904,  vol.  vii.,  p.  4). 

8  W.  H.  Pickering,  "The  Chances  of  Collision  with  a  Comet" 
(Popular  Astronomy,  No.  166,  1909,  p.  7). 

9  T.  C.  Chamberlin  and  R.  D.  Salisbury,  "Geology"  (London, 
1906),  vol.  ii.,  p.  37. 


CHAPTER  III 

THE  GEOLOGY  OF  THE  INNER  EARTH 

THE  part  of  the  earth's  crust  which  the  geologist  can 
study  by  direct  observation  and  by  the  actual  handling  of 
specimens  is  very  thin.  The  deepest  mines  are  only  about 
5,000  feet  deep ;  and,  though  some  bore-holes  have  pene- 
trated a  few  thousand  feet  lower,  they  do  not  go  below  the 
rocks  which  we  can  meet  on  the  surface  of  the  earth.  A 
depth  of  two  miles  is  insignificant  compared  with  the 
4,000  miles  to  the  earth's  centre ;  so  that  if  knowledge  of 
the  materials  of  the  earth  were  limited  to  those  we  could 
collect  and  handle,  geology  would  be  only  skin  deep,  and 
we  should  know  nothing  of  the  great  internal  mass  or  the 
deep-seated  processes  which  control  the  surface  features. 
Fortunately,  however,  the  nature  of  the  interior  of  the 
earth  is  revealed  to  us  by  many  indirect  methods. 

We  saw  in  the  last  chapter  that  the  earth  has  probably 
been  formed  by  an  aggregation  of  meteorites;  and  the 
composition  of  the  earth  is  therefore  probably  the  same 
as  that  of  meteorites. 

Most  of  the  meteorites  which  we  see  at  night  flashing 
across  the  sky  as  shooting-stars  are  burnt  up  by  friction 
with  the  earth's  atmosphere ;  but  some  of  them  fall  on  the 
earth  and  thus  allow  us  to  determine  their  actual  com- 
position. Meteorites  are  of  two  main  kinds — the  metallic 
meteorites,  which  consist  of  masses  of  iron  alloyed  with 
nickel,  and  the  stony  meteorites,  which  are  composed  of 
materials  similar  to  those  in  the  rocks  of  the  earth's  crust. 

52 


The  Geology  of  the  Inner  Earth 

The  materials  of  the  stony  meteorites  have  been  often 
broken  and  the  fragments  reunited  into  a  compact  mass, 
just  as  rocks  on  the  earth  have  been  broken  during  earth 
movements.  There  is  nothing  on  the  earth's  surface 
similar  to  the  metallic  or  nickel-iron  meteorites ;  but  there 
is  good  reason  to  believe  that  the  interior  of  the  earth  is 
largely  composed  of  such  material.  Newton  estimated 
that  the  earth  weighs  between  five  and  six  times  as  much 
as  a  globe  of  water  of  the  same  size ;  but  the  rocks  of  the 
crust  on  an  average  only  weigh  two  and  a  half  times  as 
much  as  an  equal  bulk  of  water.  Hence  Newton  knew 
that  the  earth  weighs  more  than  twice  as  much  as  it 
would  do  if  it  consisted  entirely  of  the  same  rocks  as  those 
that  form  the  surface. 

Newton's  estimate  has  been  proved  by  the  actual 
weighing  of  the  world.  This  feat  was  first  achieved  in 
1774 ;  it  was  found  that  a  plumb-line  suspended  near  the 
foot  of  Mount  Schiehallion,  in  the  Grampians,  was  attracted 
towards  the  mountain.  On  a  wide  plain  the  plumb-line 
would  have  hung  exactly  vertical ;  but  when  hung  at  the 
foot  of  Schiehallion  the  plumb-line  was  drawn  towards 
the  mountain,  and  this  movement  from  the  vertical 
position  measured  the  relative  attractions  of  the  mountain 
and  of  the  earth.  The  weight  of  the  mountain  was  then 
determined  by  multiplying  its  bulk  by  the  weight  of  the 
rocks,  and  this  result  was  then  used  to  calculate  the 
weight  of  the  world.  Such  observations  have  been  repeated 
elsewhere,  and  the  weight  of  the  world  is  regarded  as 
being  about  five  and  two-thirds  times  the  weight  of  an 
equal-sized  globe  of  water,  so  that  Newton's  estimate  has 
been  fully  confirmed. 

The  interior  of  the  world,  therefore,  consists  of  much 
heavier  material  than  the  crust.  Two  explanations  of  the 
great  weight  of  the  interior  have  been  offered.  According 
to  one,  the  composition  of  the  earth  is  uniform  throughout ; 

53 


The  Geology  of  the  Inner  Earth 

but  the  material  in  the  core  is  heavier  than  that  on  the 
surface,  as  it  is  compressed  by  the  overlying  materials. 

Laplace  estimated  that  the  central  core  of  the  earth, 
owing  to  this  cause,  would  weigh  ten  and  three-quarter 
times  as  much  as  an  equal  volume  of  water ;  and  it  has 
been  maintained  from  calculations  by  Schlichter,  that  the 
compression  of  the  interior  by  the  weight  of  the  overlying 
material  is  sufficient  to  account  for  its  greater  weight. 

According  to  the  second  and  more  probable  theory,  the 
heaviness  of  the  interior  of  the  earth  is  due  to  its  com- 
position. It  is  thought  that  the  central  mass  of  the  earth 
consists  mainly  of  metals,  while  the  crust  consists  mainly 
of  stony  materials.  The  earth  has  therefore  been  divided 
into  two  distinct  zones — a  central  metallic  mass,  known  as 
the  "  barysphere,"  or  heavy  sphere,  and  a  surrounding 
shell  known  as  the  "  lithosphere,"  or  stony  sphere.  If  so, 
then  the  iron  meteorites  would  probably  agree  in  com- 
position with  the  material  of  the  barysphere,  and  their 
study  is  enhanced  in  interest,  as  they  reveal  the  nature  of 
the  earth's  core.  As  the  barysphere  is  larger  than  the 
lithosphere,  we  should  expect  the  metallic  meteorites  to 
be  more  abundant  than  the  stony  meteorites,  and  such  is 
the  case.  Thus,  in  the  collection  of  meteorites  in  the 
Mineralogical  Department  of  the  British  Museum,  the 
weight  of  the  iron  meteorites  is  thirteen  times  as  great 
as  that  of  the  stony  meteorites.  According  to  Dr.  L. 
Fletcher's  list  of  the  meteorites  in  the  British  Museum  up 
to  1904,  the  iron  meteorites  weighed  11,873  pounds,  and 
the  stony  meteorites  only  865  pounds. 

The  preponderance  of  the  iron  meteorites  may  not  be 
generally  true,  since  the  greater  weight  of  the  iron 
meteorites  in  museums  is  largely  due  to  a  few  gigantic 
specimens ;  and  as  the  iron  meteorites  are  more  easily 
recognized  than  the  stony  meteorites,  the  collections  in 
museums  do  not  give  a  safe  test  of  their  natural  pro- 

54 


The  Geology  of  the   Inner  Earth 

portions.  It  has  therefore  been  proposed  to  compare  the 
weights  only  of  these  specimens  which  have  been  seen  to  fall. 
The  British  Museum  collection  includes  319  specimens  of 
which  the  fall  was  recorded ;  of  these  305  specimens 
weighing  802  pounds  are  stony  meteorites,  9  specimens 
weighing  2o|  pounds  are  iron  meteorites,  and  5  speci- 
mens weighing  270  pounds  were  meteorites  composed  of  a 
mixture  of  the  metallic  and  stony  constituents.  According 
to  this  test,  the  stony  meteorites  would  be  the  more 
numerous,  for  they  appear  to  fall  the  more  often ;  but  the 
metallic  meteorites  fall  in  occasional  large  masses,  some- 
times tons  in  weight,  which,  when  all  the  known  meteorites 
are  included,  outbalance  the  showers  of  small  stony 
meteorites. 

It  would  appear,  therefore,  that  nickel-iron  is  more  • 
abundant  than  stony  materials  in  meteorites.  It  is 
probably  the  more  plentiful  material  in  space.  We 
should  therefore  expect  the  barysphere  to  be  of  larger 
mass  than  the  lithosphere.  This  conclusion  is  supported  • 
by  the  evidence  of  radio-activity.  Various  substances, 
such  as  radium,  gave  forth  rays  of  heat,  and  are  therefore 
said  to  be  radio-active.  These  radio-active  substances* 
belong  to  two  series ;  the  metal  uranium  is  the  parent  of 
one  series  which  includes  radium ;  the  metal  thorium  is 
the  parent  of  another  series.  These  materials  are  con- 
stantly altering  to  other  materials,  and  the  alteration  is 
accompanied  by  the  giving  forth  of  heat.  The  earth  itself 
is  radio-active;  and  the  search  for  the  source  of  this  energy 
has  revealed  the  fact  that  the  crust  is  so  rich  in  radio- 
active materials  that  it  is  strange  the  earth's  radio-activity 
is  not  much  greater  than  it  is.  Prof.  Strutt  therefore 
concludes  that  the  radio-active  substances  in  the  earth  are 
confined  to  the  outer  crust,  and  are  all  within  forty-five  miles 
of  the  surface;  and  that  the  materials  below  that  depth 
must  be  free  from  radium.  The  nickel-iron  meteorites  are  . 

55 


The  Geology  of  the  Inner  Earth 

among  the  few  minerals  that  are  not  radio-active.  Hence 
according  to  Prof.  Strutt  the  evidence  of  radio-activity 
shows  that  there  is  a  great  change  in  the  nature  of  the 
earth  at  the  depth  of  about  forty-five  miles;  above  that 
depth  is  the  radio-active  crust,  and  below  is  the  barysphere, 
which  is  not  radio-active  because  it  is  composed  of  nickel- 
iron. 

It  is  true,  however,  that  these  conclusions  are  rejected 
by  Prof.  Joly,1  the  geologist  who  has  given  most  attention 
to  the  bearing  of  radio-activity  on  geological  problems. 
He  holds  that  radium  cannot  be  confined  to  the  outer 
zone  of  the  earth.  Radium  is  a  product  of  the  heaviest  of 
metals,  uranium,  and  the  absence  of  radium  from  the 
interior  of  the  earth  has  been  regarded  as  indicating  the 
absence  of  uranium  there.  And,  according  to  Prof.  Joly, 
"  it  is  unlikely  that  so  considerable  a  quantity  of  uranium 
should  occur  in  the  surface  rocks,  no  matter  how  deep- 
seated  their  origin,  and  not  exist  far  into  the  interior, 
if  not  to  the  very  centre." 

Prof.  Joly's  arguments  rest  on  general  considerations, 
including  the  infinite  diffusibility  of  all  known  matter. 
"  Ultimately,  indeed,"  he  remarks,  "  it  is  probably  true 
that  every  element  is  everywhere,"  which  is  a  chemical 
equivalent  of  the  logical  paradox  that  as  space  is  infinite, 
every  point  in  space  is  the  centre  of  space. 

The  conclusion  that  there  is  a  great  change  in  the 
composition  of  the  earth  at  the  depth  of  about  forty-five 
miles  is  further  supported  by  the  evidence  of  earthquakes. 
The  nature  and  origin  of  earthquakes  will  be  considered 
in  a  later  chapter;  but  their  evidence  as  to  the  internal 
structure  of  the  earth  may  be  conveniently  considered 
here. 

An  earthquake  is  a  vibration  which  travels  through  the 
crust  of  the  earth  like  a  wave  on  a  pool  of  water.  If  the 
crust  were  very  thick,  earthquakes  might  be  formed  at  a 

56 


The  Geology  of  the  Inner  Earth 

great  depth  within  or  below  the  crust.  But  though  none 
of  the  methods  used  to  determine  the  depth  at  which 
earthquakes  are  caused  give  very  precise  results,  it  is 
generally  agreed  that  they  all  originate  within  a  few  miles 
of  the  surface.  If  the  earthquake  were  due  to  something 
that  happened  deep  below  the  surface,  as  at  O  (Fig.  3), 
then  the  large  area  between  a  and  b  would  be  all  equally 
affected;  but  most  earthquakes  do  serious  damage  only 
over  a  very  limited  area,  such  as  between  q  and  r,  which 
would  be  affected  by  a  shock  arising  at  P ;  and  the  severity 
of  the  shock  decreases  quickly  beyond  q  and  r ;  while  even 
in  the  greatest  earthquakes  the  most  severely  shaken  area 


FIG.  3.— DIAGRAM  ILLUSTRATING  THE  SHALLOW  ORIGIN  OF 
EARTHQUAKES. 

is  in  general  comparatively  small.  It  is  therefore  believed 
that  earthquakes  are  always  formed  within  a  few  miles  of 
the  surface.  Yet  earthquakes  give  important  evidence 
as  to  the  interior  of  the  earth.  An  earthquake  is  a  wave 
which  travels  through  the  crust  of  the  earth,  and  causes 
the  particles  to  vibrate.  A  heavy  blow  on  the  ground 
causes  a  tremor,  which  travels  outward  from  the  point 
where  the  blow  falls,  just  as  a  wave  can  be  seen  to  travel 
outward  over  a  sheet  of  water  into  which  a  stone  has 
been  thrown. 

A  tremor  goes  quicker  through  a  dense  than  through' 
a  loose  material.    Thus  sound  travels  more  quickly  through 
water  than  through  air,  because  water  is  denser  than  air. 

57 


The  Geology  of  the  Inner  Earth 

If  an  iron  tank  partly  full  of  water  is  disturbed  by  a  slight 
explosion  or  blow  on  one  side,  the  tremors  produced  will 
travel  independently  to  the  opposite  side  of  the  tank 
through  the  iron,  through  the  water,  and  through  the  air  ; 
and  the  tremors  through  the  iron  will  reach  the  opposite 
side  first,  and  those  through  the  air  will  arrive  there  the 
last  of  the  three  series. 

Earthquakes  are  caused  by  sudden  disturbances  of  the 
crust  of  the  earth.  And  the  earthquake  travels  as  a  wave 
outward  from  the  place  of  disturbance,  which  is  known  as 
the  "  origin."  If  the  crust  of  the  earth  were  of  uniform 
material,  then  the  wave  would  travel  at  an  equal  speed  in 
all  directions.  And  all  localities  the  same  distance  from 


FIG.  4. —  DIAGRAM  ILLUSTRATING  THE  GREATER  LENGTH  OF  AN 
EARTHQUAKE  ROUTE  ALONG  THE  ARC  OBC  THAN  THROUGH 
THE  EARTH  STRAIGHT  FROM  0  TO  C. 

•  the  origin  would  feel  the  shock  at  the  same  time.  But  as 
the  crust  is  composed  of  rocks  of  variable  strength,  the 
earthquake  travels  faster  in  some  directions  than  in  others, 
and  thus  a  line  joining  places  which  feel  the  earthquake  at 
the  same  instant  of  time  is  usually  oval  and  not  circular. 

As  the  study  of  earthquakes  progressed,  it  was  found 
that  earthquakes  were  felt  at  distant  localities  sooner  than 
would  have  been  expected  from  the  rate  at  which  the 
earthquake  was  travelling  through  the  crust.  The  arrival 
of  the  earthquake  before  the  expected  time  was  easily 
explained  as  due  to  the  wave  passing  in  a  straight  line 
ODC,  through  the  earth,  instead  of  following  the  longer 
course  around  the  surface  along  OBC  (Fig.  4). 

58 


The  Geology  of  the  Inner  Earth 

If  an  earthquake  starts  at  the  point  0,  and  travels  along 
the  surface  towards  B  at  the  rate  of  sixty  miles  a  minute, 
then  it  will  be  felt  at  later  times  all  along  the  line  from 
O  to  B  and  C ;  but  it  may  be  felt  at  C  earlier  than  was 
expected  from  the  rate  of  its  journey  from  0  to  OB, 
because  (Fig.  5)  it  travels  through  the  earth  along  the 
straight  line  from  0  through  the  earth  to  C,  instead  of 


FIG.  5.— DIAGRAM  EXPLAINING  THE  QUICKENING  OF  AN  EARTHQUAKE 
WAVE  ON  ITS  ROUTE  FROM  0  TO  D  OR  E,  OWING  TO  ITS  PASSAGE 

THROUGH  THE  GEITE  CORE  OF  THE  EARTH. 

following  the  longer  line  along  the  surface  through  OBC. 
But  if  the  earthquake  be  felt  at  localities  D  and£,  so  distant 
from  the  point  of  origin  of  the  earthquake  that  the  straight 
lines  joining  the  two  places  to  the  origin  lie  deep  within 
the  earth,  then  it  is  found  that  the  earthquake  wave 
travels  through  the  earth  at  a  much  greater  speed  than  its 
rate  of  advance  through  rocks  near  the  surface.  It  will 

59 


The  Geology  of  the  Inner   Earth 

shake  D  and  E  sooner  than  it  would  if  it  travelled  at  the 
same  rate  as  along  the  line  OC.  The  late  Prof.  Milne 
estimated  from  many  observations  that  an  earthquake 
wave  travels  three  times  as  fast  through  the  central 
core  of  the  earth  as  it  does  through  the  crust.  He  ex- 
plained the  increased  speed  on  the  ground  that  the  interior 
of  the  earth  is  composed  of  a  much  denser  material  than 
the  crust.  He  called  the  central  material  "  geite,"  as  it 
forms  the  main  mass  of  the  earth. 

Hence  in  Fig.  5  the  earthquake  shock  reaches  the  sta- 
tions D  and  E  at  a  higher  speed  than  it  reaches  B  and  C, 
because  its  rate  has  been  quickened  on  passing  through 
the  dense  metallic  core  of  geite.  Hence  by  determining 
for  a  series  of  earthquakes  the  depth  at  which  the  increase 
of  speed  begins,  the  depth  of  the  surface  of  the  geite  core 
has  been  estimated.  According  to  the  calculations  by 
Milne,  the  stony  crust  of  the  earth  is  forty  miles  thick.2 
Below  is  the  core  of  geite,  which  is  the  material  of  the 
barysphere. 

1  J.  Joly,  "  Radioactivity  and  Geology,"  1909,  pp.  165-168. 

a  According  to  the  distinguished  German  authority  Wiechert,  the 
change  from  the  rocky  crust  to  geite  takes  place  at  the  depth  of 
from  800  to  1,000  miles. 


I 


60 


CHAPTER  IV 

THE  MATERIALS  OF  THE  EARTH'S  CRUST 

WE  have  seen  in  the  last  chapter  that  the  earth  consists 
of  a  large  metallic  sphere  composed  of  nickel-iron,  which 
is  about  8,000  miles  in  diameter,  and  is  surrounded  by 
a  comparatively  thin  rocky  crust.  It  is  with  this  crust  that 
the  geologist  is  mainly  concerned.  It  is  certain  that  the 
interior  of  the  earth  is  intensely  hot,  and  as  it  cools 
towards  the  surface  the  stony  matter  which  is  present 
in  the  nickel-iron  is  gradually  separated  and  is  forced 
outwards  to  the  surface.  When  ore  is  melted  in  a  furnace, 
the  heavy  metallic  portion  may  collect  as  a  mass,  known 
as  the  "  matte,"  while  the  lighter  material  is  separated 
and  solidifies  as  the  slag.  In  the  purification  of  iron  by 
the  process  known  as  "  puddling,"  the  stony  material  rises 
to  the  surface  of  the  hot  viscous  mass  of  metal,  and  there 
forms  a  scale  of  slag.  A  similar  separation  probably  hap- 
pened with  the  early  earth.  As  the  metallic  mass  cooled, 
the  stony  materials  mixed  with  it  were  pressed  to  the 
surface,  and  solidified  as  a  slag,  which  formed  the  first 
crust  of  the  earth.  Since  this  original  crust  all  passed 
through  a  molten  stage,  it  would  have  consisted  entirely 
of  the  rocks  belonging  to  the  group  known  as  "  igneous." 
As  the  crust  cooled,  the  moisture  and  gases  contained  in 
it  would  have  escaped  to  the  surface ;  the  moisture  would 
in  time  condense  into  water,  which  would  collect  in  the 
hollows  on  the  surface  of  the  crust,  and  thus  form  seas 

61 


The  Materials  of  the  Earth's  Crust 

and  lakes.  The  gases  would  remain  surrounding  the 
earth  as  its  atmosphere. 

Hence  the  changes  that  accompanied  the  slow  cooling 
of  the  earth  led  to  its  development  as  an  approximately 
spherical  body,  composed  of  four  layers.  In  the  interior 
is  the  great  metallic  core,  or  barysphere ;  on  the  surface  is 
the  rocky  crust,  or  lithosphere  ;  the  water  which  rests  on 
the  earth  in  seas,  lakes,  and  rivers,  and  those  subterranean 
waters  which  are  included  in  its  outermost  layers  and 
nourish  springs,  together  form  the  water-sphere,  or  hydro- 
sphere ;  and  outside  the  whole  is  the  zone  of  gas,  or  the 
atmosphere. 

As  soon  as  this  arrangement  was  completed,  the  waters 
and  the  atmosphere  must  have  begun  to  attack  the  surface 
of  the  lithosphere.  The  exposed  rocks  would  decay  and 
crumble,  and  the  loose  particles  would  then  be  washed 
away  by  rain  and  rivers,  or  blown  away  by  the  wind. 
This  rock  debris  would  collect  in  depressions,  and  some 
of  these  deposits  would  be  ultimately  cemented  into  new 
rocks.  These  rocks,  composed  of  second-hand  material, 
are  called  "  secondary  rocks  ";  and  as  the  igneous  rocks 
were  the  first  made,  they  are  known  as  "  primary  rocks." 

The  first  classification  of  rocks  is  into  these  two  chief 
divisions,  which  may  be  distinguished  by  three  characters : 

PRIMARY  ROCKS. 

i.  The  Primary  rocks  consist  entirely  of  crystalline 
materials,  or  of  natural  glass,  or  of  a  mixture  of  the  two, 
according  to  the  conditions  under  which  the  molten  rock 
material  cooled.  They  are  therefore  said  to  be  "  igneous." 
There  is  no  one  recognized  equivalent  term  for  all  the 
Secondary  rocks ;  some  of  them  are  made  up  of  fragments 
of  other  rocks,  and  are  called  "clastic,"  from  a  Greek 
word  meaning  broken.  Others  consist  of  material  made 
by  animals  or  plants,  and  are  known  as  "  organic"  rocks. 

62 


The  Materials  of  the  Earth's  Crust 

And  a  third  kind  have  been  deposited  by  chemical  pro- 
cesses, and  are  called  "chemically  formed"  rocks. 

2.  As  the  Primary  rocks   have   been  formed  at   high 
temperatures,  no  living  beings  were  present  in  them  at 
their  formation.    It  is  therefore  useless  to  search  in  Primary 
rocks  for  any  of  those  remains  of  animals  or  plants  which 
are  known  as  "  fossils,"  from  the  Latin  word  fossilia,  mean- 
ing "  things  dug  up."     The  term  "  fossil "  was  originally 
used  to  include  anything  dug  up  from  beneath  the  surface 
of  the  earth ;  and  thus  lumps  of  mineral,  crystals,  or  old 
tobacco-pipes,  were  all  formerly  known  as  fossils  ;  but  the 
term  is  now  restricted  to  the  buried  remains  of  animals  or 
plants.     The  Primary  rocks  are  therefore  unfossiliferous. 
The  Secondary  rocks,  on  the  other  hand,  have  been  laid 
down  in  water  or  on  land ;  so  they  often  contain  the  fossil 
remains  of  the  animals  or  plants  that  lived  at  the  time  of 
their  deposition.     The  Secondary  rocks  are  therefore  said 
to  be  fossiliferous. 

3.  Finally  the  Primary  rocks  are  said  to  be  unstratified, 
because  they  have  not  been  laid  down  in  successive  beds, 
or  "  strata "   (Latin,  stratum,  a  layer).      The  Secondary 
rocks,  on  the  other  hand,  are  described  as  "  stratified," 
because  they  have  been  laid  down  in  a  series  of  wide- 
spread layers,  or  strata;  and  this  laminated  or  stratified 
appearance   of  the   rocks   can   be   generally   seen   if  we 
examine  a  quarry  or  railway  cutting  excavated  in  Second- 
ary rocks. 

SECONDARY  ROCKS. 

The  Secondary  rocks  include  many  of  the  commonest 
and  most  widely  distributed.  They  form  the  largest  pro- 
portion of  the  surface  in  England  and  many  other  countries. 
They  are  classified  on  three  lines,  and  each  classification 
is  of  use  for  different  purposes. 

The  classification,   according  to  their  composition,  is  . 

63 


The  Materials  of  the  Earth's  Crust 

the  most  useful  for  practical  purposes,  as  the  economic 
value  of  a  rock  depends  mainly  upon  its  composition. 

» The  classification,  according  to  mode  of  formation,  distin- 
guishes those  which  were  laid  down  on  the  land  as  the  sub- 
aerial  rocks,  and  those  laid  down  in  water  as  the  aqueous 
rocks  ;  and  the  later  section  includes  the  lacustrine  rocks, 
which  have  been  deposited  in  lakes,  and  the  marine  rocks, 
which  have  been  deposited  on  the  floor  of  the  sea. 

The  third  classification  is  based  on  the  age  of  the  rocks, 
which  is  usually  determined  by  the  fossils  ;  and  this  ex- 
planation is  useful  in  working  out  the  geological  structure 
of  a  country,  and  the  relations  between  the  different  rocks 
of  which  it  is  composed.  The  whole  of  historical  geology 
is  based  on  the  classification  of  rocks  by  age. 

The  Secondary  rocks  are  divided  according  to  composi- 
tion into  three  main  series — the  sandstones,  the  clays,  and 
the  limestones.  The  sandstones  are  composed  of  grains 
of  sand  which  are  particles  sufficiently  large  to  be  easily 
recognized  by  the  naked  eye.  Sandstones  are  composed 
of  sand  grains  which  have  been  cemented  so  that  the 
material  is  firm  and  coherent.  If  the  grains  are  larger  than 
in  an  ordinary  sandstone,  the  rock  is  known  as  a  "  grit"; 
if  the  separate  particles  are  as  large  as  pebbles,  the  rock  is 
known  as  a  "  conglomerate  "  if  the  pebbles  are  rounded,  or 
as  a  "  breccia  "  (from  the  Italian  word  breccia,  which,  ac- 
cording to  Fanfani's  Dictionary,  1891,  means  "the  ruins 
of  walls,"  or  the  fragments  of  broken  masonry)  if  they  are 
composed  of  angular  fragments. 

The  sandstones  are  of  great  service  on  account  of  their 
strength  and  hardness.  They  form  the  most  resistant  of 
building  stones,  and  are  used  as  grindstones  and  paving- 
stones.  The  sandstones  found  in  the  British  Isles  are 
generally  composed  of  small  particles  of  quartz  or  silica. 

The  clays  are  also  composed  of  small  particles  which 
have  been  derived  from  Primary  rocks,  but  the  particles  are 


The  Materials  of  the  Earth's  Crust 

so  small  that  they  cannot  be  seen  by  the  naked  eye.  They 
are  less  than  one  five-thousandth  of  an  inch  in  diameter. 
The  essential  feature  of  clay  is  that  when  the  particles 
are  moistened,  the  material  becomes  plastic,  so  that  it  can 
be  moulded.  Clay  is  therefore  used  for  making  earthen- 
ware, pottery,  and  bricks. 

The  limestones  are  composed  of  carbonate  of  lime,  and 
most  of  them  have  been  formed  by  the  agency  of  animals 
and  plants  which  have  the  power  of  extracting  lime  from 
water  in  order  to  form  their  shells,  skeletons,  or  stems. 
After  the  death  of  the  animal  or  plant  the  hard  parts  re- 
main, and  accumulate  as  shell-beds,  or  in  coral  reefs,  or 
layers  of  other  calcareous  material,  which  may  ultimately 
be  cemented  into  a  firm  rock,  and  thus  form  limestone. 


DEEP-SEA  DEPOSITS. 

The  deposits  on  the  floors  of  the  deep  ocean  basins  are 
very  different  in  character  from  those  laid  down  near  the 
shores  or  on  the  surface  of  the  continents.  The  oceanic 
deposits,  having  been  formed  far  from  land,  contain  very 
little  coarse-grained  material.  The  beds  which  are  accu- 
mulating on  "  the  great  grey  level  plains  of  ooze,  where  the 
shell-burr'd  cables  creep,"  are  of  two  chief  kinds.  The 
first  kind  are  oozes,  which  are  mainly  composed  of  the 
remains  of  minute  animals  and  plants  which  lived  in  the 
ocean  far  from  land.  The  oozes  are  very  fine-grained 
white  or  grey  muds.  The  second  group  of  deep-sea  deposits 
are  red  clays,  which  are  composed  of  fragments  of  pumice 
and  grains  of  dust  which  have  been  carried  far  out  to  sea 
and  there  decayed.  The  deep-sea  deposits  must  accu- 
mulate with  extreme  slowness,  for  trawls,  which  collect 
only  from  the  surface  layer,  bring  up  abundant  ear-bones 
of  whales  and  the  teeth  of  extinct  sharks  which  must  have 
died  many  millenniums  ago;  but  as  the  ear-bones  and  teeth 

65  E 


The  Materials  of  the  Earth's  Crust 

are  not  readily  dissolved,  they  remain,  but  have  not  yet 
been  covered. 

Different  forms  of  ooze  are  deposited  according  to  the 
depth  and  temperature  of  the  sea  and  other  factors. 
It  was  expected  that  the  same  type  of  ooze  would  be 
deposited  at  the  same  place  for  a  long  period,  and  thus 
form  deposits  of  great  thickness.  This  question  was  first 
put  to  the  proof  by  the  German  Antarctic  Expedition  of 
1901-1903.  By  a  special  apparatus  it  collected  samples  two 
or  three  feet  thick  of  deep-sea  deposits,  and  it  thus  showed 
that  the  different  kinds  of  ooze  are  interbedded  in  thin 
layers.  Thus  a  sample  collected  from  the  depth  of  17,100 
feet  from  a  locality  in  the  South  Atlantic  to  the  west  of 
Cape  Colony,  showed  the  following  variations  : 

i$  inches  =  red  clay  (with  grains  of  igneous  rocks). 

if  inches  =  calcareous  ooze  (Globigerina). 

10  inches  =  red  clay. 

15  inches  =  Globigerina  ooze  (containing  30  to  50  per  cent. 

of  carbonate  of  lime). 
3^  inches  =  red  clay. 

The  origin  of  the  sedimentary  rocks  is  so  simple  and 
obvious  that  the  chief  problems  connected  with  them  were 
soon  settled.  In  recent  years  improved  methods  have 
been  developed  for  determining  the  geographical  condi- 
tions under  which  these  rocks  were  formed,  for  tracing 
the  source  whence  their  materials  came,  and  for  investi- 
gating the  deposits  laid  down  at  great  depths  on  the 
ocean  floors. 

METAMORPHIC  ROCKS. 

The  main  difficulties  and  the  most  interesting  problems 
in  the  study  of  rocks  are  connected  with  the  Primary  and 
metamorphic  rocks.  Thirty  years  ago  geologists  were 
mostly  concerned  with  the  metamorphic  rocks,  which  in 
many  respects  are  intermediate  between  the  Primary  and 
Secondary  groups.  They  agree  with  Secondary  rocks  by 

66 


The  Materials  of  the  Earth's   Crust 

occurring  in  layers,  so  that  they  were  obviously  laid  down 
in  a  stratified  series ;  and  they  agree  with  most  Primary 
rocks  in  the  crystalline  nature  of  their  materials.  This 
crystalline  character  is  not  an  original  property  of  the 
rock ;  it  has  been  produced  by  the  crystallization  of  its 
constituents  under  the  influence  of  great  heat  or  immense 
pressure,  or  of  percolating  waters. 

An  unbroken  passage  can  be  traced  from  comparatively 
unaltered  sedimentary  rocks,  in  which  the  original  grains 
are  quite  distinct,  to  metamorphic  rocks,  which  consist 
wholly  of  simple  minerals  that  have  crystallized  in  the 
rock,  and  in  some  cases  these  simple  minerals  are  the 
same  as  in  the  Primary  rocks.     A  famous  example  of  this 
passage  may  be  seen  around  the  granite  of  Skiddaw  in  the 
Lake  District.     The  granite  there  has  been  intruded  into 
the  slates,  forming  the  main  mass  of  the   mountain  of 
Skiddaw.     These  slates  were  no  doubt  originally  deposited 
as  clays  on  a  sea-floor.     The  rocks  have  been  so  squeezed 
and  compressed  that  they  are  metamorphosed,   and  the 
resultant  rock  now  breaks   into   thin  smooth  slabs,  and 
has  therefore  become  a  slate,  in  which,  on  close  examina- 
tion, the  original  grains  may  be  still  discerned.   Approach- 
ing the  margin  of  the  granite  the  slate  becomes  harder, 
and   is   seen   to   be   undergoing   alteration.     Dark   spots 
appear    on    the    smooth    surfaces,   which   then   become 
glistening  and  silvery,  owing  to  the  abundance  of  minute 
white  flakes.     These  increase  in  size,  and  are  recognizable 
as  white   mica,  the   scales  of  mica   being  separated  by 
layers  of  fresh  quartz.     No  trace  of  the  original  grains 
can  be  observed.     The  rock  has  thus  become  a  "  schist." 
Stili  closer  to  the  edge  of  the  granite  some  of  the  common 
rock,  forming  a  simple  mineral  known  as  "  felspar,"  appears 
in  addition  to  the  quartz  and  mica.     Hence  specimens 
can  be  obtained  which  consist  of  the  same  three  simple 
minerals,  as  in  granite  ;  but  the  rock  differs  from  granite 


The  Materials  of  the  Earth's  Crust 

by  its  constituents  being  arranged  in  lines  (or  folia), 
instead  of  being  mixed  irregularly.  Owing  to  this  regular 
arrangement  of  the  constituents,  the  rock  is  said  to  be 
foliated,  from  the  Latin  folium,  a  leaf,  and  to  distinguish  it 
from  granite  it  is  called  "  gneiss."  This  gneiss  has  been 
produced  from  slate  owing  to  the  influence  of  the  invading 
granite,  some  of  the  materials  of  which  entered  the 
adjacent  rock,  and  thus  helped  its  conversion  into  gneiss. 

The  passage  from  sedimentary  rocks  to  schist  and 
gneiss  may  be  observed  in  many  parts  of  the  world,  and 
ordinary  secondary  rocks  may  be  so  altered  as  to  consist 
of  the  same  mineral  species  as  Primary  rocks.  It  was 
suggested  that  many  Primary  rocks  were  due  to  one 
further  stage  in  this  process.  Thus,  granite  might  be 
a  Secondary  rock,  which  had  been  so  heated  and  altered 
that  all  the  original  grains  had  been  recrystallized,  and 
had  lost  all  trace  of  their  secondary  origin. 

At  Skiddaw  the  band  of  schist  and  gneiss  is  compara- 
tively narrow,  but  in  other  parts  of  the  world  metamorphic 
rocks  occur  in  continuous  sheets,  covering  many  thou- 
sands of  square  miles.  These  large  areas  of  metamorphic 
rocks  may  have  been  formed  by  the  alteration  of  sedi- 
mentary rocks  by  a  somewhat  similar  process ;  only  in 
such  cases  the  heat  has  not  been  due  to  the  intrusion 
of  a  mass  of  granite,  but  to  the  Secondary  rocks  having 
sunk  so  deep  below  the  earth's  surface  that  they  have 
been  greatly  affected  by  the  internal  heat  of  the  earth. 
The  conversion  of  Secondary  into  metamorphic  rocks  on 
the  margin  of  a  molten  rock  is  known  as  "  contact  meta- 
morphism."  Rocks  are  such  poor  conductors  of  heat  that 
when  only  at  a  small  distance  from  molten  rock  they 
may  be  very  slightly  influenced  by  its  heat.  Hence,  con- 
tact metamorphism  is  confined  to  narrow  bands  of  rock, 
and  it  cannot  explain  the  formation  of  vast  areas  of  meta- 
morphic rocks.  As  the  rocks  of  a  wide  area  sink  slowly 

68 


The  Materials  of  the  Earth's  Crust 

into  the  deeper  parts  of  the  earth's  crust,  the  whole  series 
gradually  becomes  intensely  heated,  and  the  metamor- 
phism  thus  produced  is  known  as  "  thermo-metamor-  . 
phism,"  since  it  is  due  to  the  earth's  internal  heat.  There 
is  now  no  doubt  that  widespread  areas  of  schist  and 
gneiss,  such  as  we  find  in  the  Scottish  Highlands,  in  large 
parts  of  eastern  Canada,  in  India  and  Africa,  are  due  to 
the  recrystallization  by  heat  of  old  Secondary  rocks.  But  • 
the  view  that  the  Plutonic  rocks  represent  a  further  stage 
of  this  metamorphism  has  not  yet  been  established,  and  is 
improbable. 

The  metamorphism  of  vast  areas  appears  to  have  been 
limited  to  the  oldest  rocks  of  the  earth's  crust.  Clays 
may  have  been  pressed  into  slates  and  the  slates  rendered 
shiny  by  the  development  of  minute  flakes  of  mica  by 
earth  movements  during  any  part  of  geological  history. 
But  so  far  as  has  yet  been  proved,  all  the  great  areas 
of  schist  and  gneiss  consist  of  rocks  which  were  formed 
in  the  first  era  of  geological  time.  It  has  therefore  been 
suggested  that  the  formation  of  widespread  areas  of  schists 
ceased  on  the  earth  before  the  deposition  of  the  oldest 
fossiliferous  rocks.  There  seems  no  reason  why  thermo- 
metamorphism  should  not  have  occurred  in  later  times, 
and  be  in  progress  still.  Many  geologists  hold  that  some 
of  the  great  bodies  of  schist  and  gneiss  have  been  formed 
by  the  alteration  of  fossiliferous  rocks,  which  were  de- 
posited at  comparatively  recent  periods  in  the  history  of 
the  earth.  But  in  many  cases  this  claim  has  been  con- 
clusively disproved,  and  it  has  not  yet  been  anywhere 
sufficiently  well  established  to  have  received  universal 
assent.  The  fact  that  most  if  not  all  the  known  areas  of 
schist  and  gneiss  are  composed  of  very  ancient  rocks 
may  be  the  inevitable  consequence  of  metamorphism  being 
a  very  deep-seated  process.  Any  younger  beds  that  had  sunk 
deep  enough  to  have  been  affected  by  thermo-metamorphism 


The  Materials  of  the  Earth's  Crust 

might  still  be  buried  out  of  reach.  A  very  long  time  must 
elapse  before  they  would  be  exposed  on  the  surface.  This 
simple  explanation  appears,  however,  inadequate. 

The  view  that  large  areas  of  fossiliferous  rocks  have 
been  altered  into  schists  and  gneiss  gave  rise  to  the  most 
famous  controversy  in  the  history  of  British  geology. 
Macculloch,  the  first  geologist  who  made  any  important 
contribution  to  the  geology  of  north-western  Scotland, 
discovered  in  1819  that  to  the  east  of  Cape  Wrath  in 
Sutherlandshire  metamorphic  and  fossiliferous  rocks  are 
apparently  interbedded.  Thus,  journeying  eastward  along 
the  shore  of  the  Pentland  Firth  from  Cape  Wrath,  the 
first  rocks  seen  are  some  coarse-grained  gneisses  ;  they  are 
covered  by  a  series  of  red  sandstones,  which,  in  turn,  are 
covered  by  hardened  sandstones  and  some  limestones, 
both  of  which  contain  well-preserved  fossils.  Still  farther 
eastward  these  fossiliferous  rocks  are  covered  by  a  second 
series  of  gneisses  and  schists.  In  accordance  with  the 
ordinary  law  that  in  a  series  of  beds  the  uppermost  are 
the  youngest,  Macculloch  concluded  that  these  eastern 
gneisses  and  schists  were  younger  than  the  fossiliferous 
beds  below  them.  The  collection  of  a  more  complete 
series  of  fossils  showed  that  the  limestones  belong  to  the 
period  known  as  the  Ordovician,  and  the  sandstones 
below  them  are  Cambrian.  The  still  older  red  sand- 
stones east  of  Cape  Wrath  are  therefore  earlier  than  the 
Cambrian,  and  the  gneisses  of  Cape  Wrath  are  the  oldest 
of  the  series. 

Sir  Roderick  Murchison,  always  alert  in  following  a 
promising  clue,  visited  the  area  in  order  to  determine 
whether  Macculloch  was  right  as  to  the  eastern  gneisses 
being  really  younger  than  the  fossiliferous  limestone ;  for 
if  so,  they  would  be  necessarily  younger  than  the  Ordo- 
vician. His  examination  of  the  area  led  him  to  confirm 
Macculloch's  view,  and  these  eastern  gneisses  have  been 

70 


The  Materials  of  the  Earth's  Crust 

traced  eastward  and  southward  and  proved  to  be  the 
northern  part  of  the  sheet  of  similar  rocks  which  form 
practically  the  whole  of  the  Highlands  of  Scotland  from 
the  Pentland  Firth  to  the  southern  edge  of  the  Highlands 
near  Glasgow.  Hence  Murchison  concluded  that  the 
Scottish  Highlands  are  composed  of  a  series  of  Silurian 
rocks,  which  had  been  altered  into  schists  and  gneiss  at 
the  end  of  Silurian  times.  The  alteration  was  certainly 
older  than  the  time  of  Old  Red  Sandstone,  which  in 
Scotland  represents  the  succeeding,  or  Devonian,  period. 
Murchison's  view  was  promptly  contested  from  the  extreme 
improbability  that  some  of  the  rocks  could  have  been 
crystallized  into  schists,  while  the  underlying  sandstones 
and  limestones  had  been  left  comparatively  unaltered. 

An  attempt  was  made  to  remove  this  difficulty  by  the 
hypothesis  of  what  was  called  "  selective  metamorphism." 
It  was  suggested  that  certain  beds  might  be  of  such  an 
unstable  composition  that  they  would  easily  become  re- 
crystallized,  while  the  associated  rocks  might  be  of  such 
a  stable  composition  that  they  would  remain  unaltered. 
The  explanation  was  never  plausible ;  for  the  altered 
series  include  crystalline  limestones  and  altered  sand- 
stones, and  there  appeared  no  reason  why  these  should 
have  been  metamorphosed  while  the  underlying  limestones 
and  sandstones  remained  unchanged.  The  superposition 
of  the  eastern  gneisses  on  the  underlying  rocks  was  so 
clear  that  for  a  generation  Sir  Roderick  Murchison's  view 
was  almost  universally  accepted.  It  was  not  until  1884 
that  Prof.  Bonney  proved  that  some  of  the  eastern  or 
supposed  younger  gneisses  were  part  of  the  western  or 
older  gneisses ;  they  were  originally  at  the  bottom  of  the 
series,  but  they  have  been  brought  to  the  surface  and 
placed  over  the  younger  rocks  by  earth  movements.  It 
was  not  until  a  few  years  later  that  some  of  the  difficulties 
in  this  explanation  were  removed  by  Prof.  Lapworth  of 


The  Materials  of  the  Earth's  Crust 

Birmingham,  who  showed  that  the  eastern  gneisses  had 
been  upraised  and  pushed  sideways  over  the  fossiliferous 
rocks;  and  the  very  puzzling  characters  of  some  of  the 
rocks  he  proved  to  be  due  to  their  having  been  ground  to 
powder  during  these  earth  movements. 

Prof.  Lapworth's  view  was  fully  confirmed  on  the 
detailed  survey  of  the  area  by  the  Geological  Survey  of 
Scotland,  and  it  is  now  known  that,  instead  of  these 
schists  and  gneisses  being  younger  than  the  limestones, 
they  are  older. 

Scotland,  therefore,  does  not  give  any  evidence  of  a  wide 
series  of  fossiliferous  rocks  having  been  altered  into  schists 
and  gneiss.  Similarly  in  the  Alps  Prof.  Bonney  has 
disproved  the  view  that  some  of  the  rocks  near  the  St. 
Gothard  are  schists  due  to  the  alteration  of  rocks  of 
about  the  same  age  as  our  coal  measures  ;  for  the  real 
schists  there  are  much  older  than  any  fossiliferous  rocks 
in  the  Alps. 

THE  IGNEOUS  ROCKS. 

After  the  close  of  the  great  controversy  on  the  rocks  of 
the  north-west  of  Scotland,  the  main  interest  has  passed 
from  the  metamorphic  to  the  igneous  rocks ;  their  classi- 
fication and  the  origin  of  the  different  varieties  is  still  the 
most  engrossing  problem  in  petrology. 

The  igneous  rocks  are  classified  according  to  two 
lines :  first,  according  to  their  composition ;  and,  secondly, 
according  to  the  different  conditions  under  which  they 
solidified.  The  two  systems  of  classification  are  important, 
both  economically  and  from  their  scientific  interest.  Both 
composition  and  the  conditions  of  solidification  affect  the 
practical  value  of  the  rocks ;  for  they  affect  their  weight, 
durability,  and  toughness.  The  conditions,  however, 
under  which  a  rock  has  solidified  produce  the  most 
striking  features  in  its  general  appearance.  If  a  molten 

72 


The  Materials  of  the  Earth's  Crust 

rock  material  has  solidified  slowly  and  at  a  great  depth, 
then  the  whole  of  it  will  be  crystalline.  This  does  not 
mean  that  all  the  constituents  are  formed  in  crystals  with 
smooth  external  surfaces  like  cut  gems  or  like  crystals  that 
may  be  seen  in  a  large  collection  of  minerals.  Crystals 
with  such  perfect  external  forms  are  usually  developed 
when  the  mineral  grows  in  a  fluid  or  in  an  open  space. 
Where  the  rock  material  has  solidified  under  great  pressure, 
the  separate  crystals  are  so  intergrown  that  their  shapes 
are  irregular. 

Sugar-candy  consists  of  crystals  of  sugar  which  have 
grown  in  a  fluid  so  that  they  can  develop  regular  external 
surfaces,  whereas  in  loaf-sugar  the  crystals  are  so  crowded 
together  that  they  form  a  tangled  mass  where  none  of  the 
crystals  develop  their  normal  external  surface;  but  the 
sugar  is  as  much  crystalline  in  the  loaf-sugar  as  in  the 
sugar-candy.  The  essential  property  of  a  crystalline 
material  is  that  its  particles  are  regularly  arranged, 
whereas  in  non-crystalline  materials  the  arrangement  of 
the  particles  is  quite  irregular.  If  a  crystal  of  quartz 
be  melted  and  cooled  quickly,  the  actual  composition 
remains  unchanged ;  but  the  material  will  have  been 
changed  in  structure ;  it  will  no  longer  be  crystalline, 
but  a  glass,  for  its  particles  will  be  arranged  higgledy- 
piggledy. 

If  toy-bricks  be  thrown  into  their  box  anyhow,  the  box 
does  not  hold  so  many  as  it  would  if  they  were  carefully 
packed  side  by  side ;  and  the  box  if  carefully  packed  would 
not  only  hold  more,  but  would  weigh  more  than  when  the 
bricks  were  simply  thrown  in.  It  is  the  same  with  molten 
quartz.  If  it  resolidify  under  conditions  which  cause  the 
particles  to  be  regularly  arranged  so  that  it  has  a  crystalline 
structure,  a  given  bulk  holds  a  larger  number  of  particles 
and  is  heavier  than  if  the  particles  had  been  arranged 
irregularly.  A  cubic  inch  of  crystalline  quartz  weighs 

73 


The  Materials  of  the  Earth's  Crust 

650  grains.  A  cubic  inch  of  quartz  which  has  melted  and 
resolidified  as  a  glass  only  weighs  560  grains.  Hence,  if 
a  rock  solidifies  under  great  pressure,  the  tendency  is  for 
all  the  particles  to  be  packed  as  closely  together  as  possible, 
and  the  whole  material  is  solidified  in  a  crystalline  condi- 
tion. If,  on  the  other  hand,  the  material  solidifies  under 
very  slight  pressure,  much  of  it  will  solidify  as  a  glass. 
Under  intermediate  conditions  part  of  the  material  may 
solidify  as  a  glass  and  the  rest  in  a  crystalline  form.  A 
rock,  such  as  granite,  which  has  solidified  at  a  great  depth 
below  the  earth's  surface  under  enormous  pressure,  is 
wholly  composed  of  crystalline  material.  It  is  said  to  be 
holocrystalline,  whereas  if  the  same  material  had  cooled 
on  the  surface  of  the  earth — say,  on  the  top  of  a  lava-flow, 
it  would  solidify  wholly  as  the  natural  glass  obsidian,  or  as 
the  glassy  froth  known  as  "  pumice."  If  the  same  material 
had  solidified  under  slight  pressure,  then  the  rock  formed 
would  consist  of  crystals  surrounded  by  glass.  The  amount 
of  glass  in  a  rock  is  therefore  an  indication  of  the  pressure 
under  which  it  solidified.  The  proportion  of  glass  cannot 
be  used  to  fix  the  depth  in  any  definite  number  of  feet ; 
but  the  rocks  which  have  consolidated  at  great  depths 
contain  no  glass,  while  lava-flows  usually  contain  much 
glass. 

The  proportion  of  glass  present  may  be  used  to  divide 
igneous  rocks  into  three  groups.  The  first  group  consists 
of  the  Plutonic  rocks — granite,  for  example — which  are 
formed  at  a  great  depth  below  the  earth's  surface,  and 
therefore  contain  no  glass. 

The  second  group  includes  the  volcanic  rocks  which 
have  been  poured  over  the  earth's  surface  by  lava-flows ; 
they  are  named  after  Vulcan,  the  divine  smith  whose 
fires  flared  forth  through  the  volcanic  vent  of  Etna  and 
whose  smoke  obscured  the  Sicilian  skies.  "  How  often," 
wrote  Virgil  ("  Georgics,"  bk.  i.,  lines  471-473),  "have  we 

74 


The  Materials  of  the  Earth's  Crust 

seen  rocking  Etna,  its  furnaces  broken,  boil  over  into  the 
fields  of  the  Cyclops,  and  roll  down  flaming  boulders  and 
molten  rocks." 

The  third  group,  including  rocks  that  occur  in  sheets 
thrust  into  other  rocks,  is  an  intermediate  series ;  its 
rocks  have  been  formed  under  greater  pressure  than  the 
volcanic  rocks,  but  under  less  pressure  than  the  Plutonic. 
The  igneous  rocks  are  also  classified  according  to  com- 
position. Some  of  them  are  dark  in  colour  and  are  heavy, 
because  they  contain  much  iron,  while  others  are  lighter 
in  weight  and  colour  owing  to  their  poverty  in  iron.  The 
light  rocks,  poor  in  iron,  contain  much  more  of  the  sub- 
stance silica,  the  constituent  of  flint,  and  some  of  this  is 
often  present  in  the  form  of  quartz,  which  can  be  recognized 
by  the  naked  eye.  The  darker-coloured  rocks  contain  so 
little  silica  that  none  of  it  is  present  in  the  form  of  quartz. 
As  silica  is  regarded  as  the  acid  element  in  rocks,  those 
containing  much  silica  are  called  "  acid  "  rocks  ;  those  in 
which  the  silica  is  much  less  in  proportion  to  the  other 
or  basic  constituents  are  therefore  known  as  the  "basic" 
rocks.  Between  these  two  extreme  groups  is  a  series  with 
an  intermediate  proportion  of  silica,  and  they  are  known 
as  the  Intermediate  group. 

The  ultimate  test  of  the  composition  of  rocks  is  chemical 
analysis  ;  but  the  full  chemical  analysis  of  a  rock  is  a  slow 
and  laborious  undertaking,  which  occupies  practically 
a  week.  Two  rock  analyses  may  be  done  at  the  same 
time,  so  that  two  may  be  completed  in  the  week. 

Fortunately,  the  approximate  chemical  composition  of 
a  rock  can  be  determined  without  so  lengthy  an  investiga- 
tion. Acid  and  basic  rocks  may  be  roughly  separated  in 
the  field  by  the  colour  which  they  take  when  weathered : 
acid  rocks  weather  into  pale  greys  ;  the  basic  rocks  become 
stained  by  iron  rust,  and  so  are  dark  brown.  A  more 
accurate  test  is  afforded  by  the  weight  of  the  rocks,  which 

75 


The  Materials  of  the   Earth's  Crust 

can  be  tested  in  a  few  minutes.  An  acid  rock,  owing  to 
its  poverty  in  iron,  weighs  less  than  an  equal  bulk  of  a 
basic  rock,  and  thus,  by  simply  weighing  the  material,  an 
approximate  estimate  of  the  amount  of  silica  is  obtained. 

A  still  more  precise  determination  of  the  composition 
of  a  rock  is  given  by  the  use  of  the  microscope.  A  small 
piece  of  the  rock  is  ground  so  thin  that  it  is  transparent, 
and  it  can  then  be  examined  under  the  microscope.  The 
various  constituents  of  the  rock  can  be  recognized,  their 
proportions  measured,  and  thus  the  composition  of  the 
rock  determined.  The  invention  of  examining  rocks  in 
thin  sections  through  the  microscope  has  led  to  a  revolu- 
tion in  their  study. 

Rocks  are  divided,  according  to  composition,  into  three 
main  divisions :'  The  acid,  in  which  the  rocks  are  rich  in 
.  silica,  and  contain  free  quartz  ;^  the  basic,  in  which  the 
rocks  are  poor  in  silica,  rich  in  basic  materials,  such  as 
iron,  magnesium,  and  lime,  and  generally  contain  the 
simple  mineral  olivine,  but  no  quartz  ;•*•  the  third,  or 
intermediate  group,  which  includes  rocks  that  are  inter- 
mediate in  composition  between  the  other  two,  and 
generally  contain  neither  quartz  nor  olivine.  Each  of 
these  three  divisions  has  a  typical  plutonic  rock ;  each 
plutonic  rock  has  a  volcanic  equivalent,  formed  when  the 
material,  which,  if  solidified  deep  in  the  earth  would  have 
formed  a  plutonic  rock,  has  reached  the  surface,  and  there 
overflowed  as  a  lava. 

Between  the  three  divisions — the  basic,  the  acid,  and  the 
intermediate — there  is  a  perfectly  gradual  transition ;  and 
there  is  a  remarkable  uniformity  in  the  constituents 
present  in  all  rocks.  It  was  early  recognized  by  the 
famous  chemist  Bunsen  that  all  the  rocks  he  knew  were 
surprisingly  similar  in  chemical  composition  ;  and  this 
fact  has  been  confirmed  by  the  subsequent  examination  of 
rocks  from  all  parts  of  the  world,  including  far  more  than 


The  Materials  of  the  Earth's  Crust 

were  known  to  Bunsen.  Igneous  rocks  are  mainly  com- 
posed of  seven  materials  :  silica,  alumina,  lime,  potash, 
soda,  magnesia,  and  oxide  of  iron.  The  chief  difference 
between  the  various  igneous  rocks  is  in  the  proportions  of 
these  constituents.  The  different  species  of  minerals,  on 
the  other  hand,  include  a  very  great  variety  of  chemical 
substances,  and  it  was  naturally  expected  at  first  that  rocks 
would  show  the  same  diversity  in  composition.  Their 
surprising  similarity  in  composition  suggested  that  they 
were  all  derived  from  the  same  source,  a  great  subter- 
ranean reservoir  of  molten  rock. 

Bunsen  found  that  there  are  two  chief  distinct  types  of 
rock,  so  he  was  led  to  the  conclusion  that  all  igneous 
rocks  are  derived  from  either  of  two  subterranean  reservoirs 
of  molten  rock  material,  or  of  mixtures  in  various  pro- 
portions of  these  two  materials.  For  these  two  types  the 
names  that  are  generally  adopted  are  the  acid  and  basic. 
The  typical  acid  plutonic  rock  is  granite,  of  which  the 
lava  equivalent  is  liparite,  so  named  because  it  includes 
the  common  lava  of  the  Lipari  Islands.  The  best-known 
rock  of  the  basic  group  is  basalt.  Mixtures  of  various 
proportions  of  molten  granite  and  molten  basalt  would 
give  rise  to  the  various  intermediate  types. 

Rocks  vary  not  only  in  the  amount  of  basic  material 
present  in  them,  but  also  in  the  amount  of  the  alkalies, 
soda  and  potash.  Some  of  them  are  very  rich  in 
these  constituents,  and  are  known  as  "  alkali-rich "  or 
"  alkali "  rocks. 

There  is,  however,  no  clear  evidence  of  the  actual 
existence  of  any  two  such  subterranean  masses,  or  layers, 
of  acid  and  basic  rock  as  Bunsen  assumed.  Hence 
proposals  have  been  made  to  explain  the  variation  in  the 
composition  of  the  rocks  by  invoking  the  two  processes 
known  as  "  differentiation  "  and  "  assimilation."  To  deter- 
mine if  differentiation,  or  assimilation — i.e.,  the  mixture  of 

77 


The  Materials  of  the  Earth's  Crust 

two  different  molten  materials — is  responsible  for  the  dif- 
ferent varieties  of  rocks,  is  the  main  problem  of  modern 
petrology. 

The  study  of  rocks  is  one  of  the  most  technical  branches 
of  geology,  and  it  can  only  be  considered  in  very  general 
terms,  where  chemical  and  physical  technicalities  are 
banned. 

It  was  remarked  in  Chapter  I.  that  some  of  the  first 
geologists  considered  even  such  rocks  as  granite  had  been 
deposited  by  water.  This  view  was  adopted  in  spite  of 
an  early  recognition  of  the  facts,  which  indicate  that 
granite  had  been  injected  as  a  liquid  into  other  rocks ; 
but  this  view  of  the  origin  of  granite  was  rejected  owing 
to  the  relations  between  the  three  constituents  of  the 
rock. 

Granite  is  composed  of  three  simple  minerals  or  mineral 
species,  which  are  known  as  "  quartz,"  "  felspar,"  and 
"  mica."  The  felspar  forms  the  largest  part  of  the  rock,  and 
in  a  piece  of  coarse-grained  granite  it  may  be  recognized  in 
large,  smooth,  white  or  pink  surfaces ;  the  felspar  can  be 
just  scratched  with  a  sharp  knife.  The  mica  occurs  in 
silvery  white,  or  in  black  or  brown  scales,  and  they  can 
be  easily  broken  by  a  knife  into  flakes  as  thin  and  smooth 
as  paper.  The  quartz  occurs  as  irregular  patches  or  grains 
that  look  like  pieces  of  ground  glass  ;  they  are  so  hard 
that  they  cannot  be  scratched  with  a  knife,  but  will  them- 
selves scratch  glass. 

Close  study  of  the  relations  of  the  three  constituents 
shows  that  the  mica  was  usually  formed  first,  then  the 
felspar,  and  finally  the  quartz  filled  up  the  interspaces 
between  the  felspar  and  mica.  Now,  if  some  quartz, 
felspar,  and  black  mica  were  melted  in  three  separate 
crucibles,  which  were  then  allowed  to  cool,  it  would  be 
found  that  the  quartz  would  become  solid  first  at  a 
temperature  of  2,600°  F. ;  the  felspar  would  solidify  next, 

78 


The  Materials  of  the  Earth's  Crust 

when  the  temperature  fell  to  2,175°  F. ;  and  the  black 
mica  wouM  solidify  last  at  the  temperature  of  2,120°  F. 

This  order  of  solidification  is  the  exact  opposite  of  that 
in  which  the  three  minerals  have  been  formed  in  granite ; 
and  this  fact  appeared  inconsistent  with  the  view  that 
granite  was  formed  as  a  molten  rock,  as  if  so  the  quartz 
should  have  formed  first  instead  of  last.  But  this  con- 
clusion was  based  on  an  altogether  wrong,  though  very 
natural,  assumption.  If  the  contents  of  the  three  crucibles 
were  mixed  together,  and  the  mixture  were  allowed  to 
cool,  then  it  would  be  observed  that  the  mica  would  be 
formed  first,  the  felspar  next,  and  the  quartz  last.  The 
order  of  solidification  in  the  mixture  would  be  the  same 
as  in  granite.  The  order  in  which  simple  minerals  are 
formed  in  a  molten  material  containing  many  ingredients 
is  not  that  of  their  melting-points,  but  of  their  solubility. 
If  a  hot  complex  solution  be  allowed  to  cool,  various 
ingredients  are  separated  from  the  rest  and  deposited  as 
crystals;  but  the  order  in  which  these  materials  are 
formed  does  not  depend  on  the  temperatures  at  which 
these  materials  would  melt  on  exposure  to  heat,  but  on 
their  solubility  in  the  solution. 

The  molten  granite  is  a  complex  solution,  consisting  of, 
say,  %  y  z,  x  representing  the  materials  of  the  quartz,  y  of 
the  felspar,  and  z  of  the  mica;  the  order  in  which  they 
would  solidify,  if  it  were  determined  by  their  melting- 
points,  is  x,  y,  z.  But  as  the  solution  cools  down,  z  first 
becomes  insoluble,  and  is  separated  as  crystals  of  mica ; 
the  constituents  of  y  solidify  next  as  felspar ;  and  the 
material  of  the  quartz,  which  has  hitherto  acted  as  the 
solvent  of  the  other  constituents,  is  then  free  to  solidify 
in  all  the  interspaces  left. 

Quartz  when  acting  as  a  solvent  will  remain  liquid  far 
below  its  normal  melting-point,  and  thus  the  sequence  of 
the  solidification  of  the  three  essential  constituents  of  the 

79 


The  Materials  of  the  Earth's  Crust 

granite  is  quite  consistent  with  the  igneous  origin  of 
granite.  Moreover,  other  minerals  may  be  present  in  a 
granite  which  cannot  exist  at  the  temperature  of  pure 
molten  quartz. 

The  sequence  of  the  solidification  of  the  constituents 
of  molten  rocks  has  such  an  important  bearing  on  the 
natural  history  of  the  rocks  that  it  is  necessary  to  consider 
this  further.     The  constituents  of  all  igneous  rocks  solidify 
in  a  similar  order  to  those  of  granite,  and  this  order  follows 
"the  law  of  decreasing  basicity."     According  to  that  law 
the   most   basic   minerals  crystallize  first,  and  the  least 
basic  last.     The  term  "  base  "  has  several  meanings :    it 
is  used  differently  in  geology,  in  chemistry,  in  dyeing,  and 
in  architecture,  or  when  used  as  the  name  of  an  apron, 
or  in  such  expressions  as  "  a  basic  principle,"  or  "  the 
base  of  a  series."     In  the  study  of  rocks  the  term  "  basic  " 
is  applied  to  those  which  are  rich  in  iron,  lime,  or  mag- 
nesium.    Silica,  the  material  of  quartz,  is  regarded  as  the 
acid  constituents  of  rocks,  and  as  quartz  consists  only  of 
silica,  it  is  classified  as  the  most  acid  of  mineral  species. 
In  most  of  the  mineral  species  which  are  abundant  in  the 
earth's  crust,  silica  is  combined  with  other  constituents, 
and  these  are  usually  composed  of  oxygen,  combined  with 
other   elements,  including  aluminium,  iron,  magnesium, 
potassium,    sodium,    and    calcium.      The   compounds   of 
oxygen  with  these  elements  are  known  as  their  "  oxides." 
The  oxide  of  calcium  is  commonly  known  as  "  lime."    The 
oxides  of  iron  and  magnesium  and  lime  are  regarded  as 
the  basic   constituents  of  rocks.     Some   mineral  species 
found  in  rocks  consist  only  of  "  basic  "  material,  just  as 
quartz   consists    only   of   the    acid   constituent.      Thus, 
magnetite,  the  magnetic  oxide  of  iron,  consists  only  of 
iron  and  oxygen,  and  it  is  wholly  "  basic."     Most  rock- 
forming  mineral  species  consist  of  some  acid  and  some 
basic  materials.     Some  have  an  excess  of  the  acid,  and 

80 


The  Materials  of  the  Earth's  Crust 

others  an  excess  of  the  basic  constituents ;  and  in  others 
the  two  types  of  material  are  evenly  balanced. 

Now,  the  law  of  decreasing  basicity  asserts  that  the 
most  basic  elements  in  a  molten  rock  solidify  first,  then 
those  which  are  the  next  richest  in  basic  materials,  and 
the  most  acid  solidify  last. 

Rock-forming  minerals  have  been  classified,  according 
to  this  sequence,  into  four  groups : 

The  first  includes  metallic  oxides,  such  as  magnetite, 
and  various  mineral  species,  which  usually  form  a  very 
small  proportion  of  the  rock. 

Second,  the  species  of  minerals  which  are  rich  in  iron 
and  magnesium,  and  are  therefore  known  as  the  "  ferro- 
magnesium  group."  To  this  group  belongs  the  bulk  of 
the  heavier  and  coloured  constituents. 

Third,  the  group  composed  only  of  silica,  alumina,  and 
the  oxides  of  the  alkalies,  potash  and  soda,  and  the  earths, 
lime  and  magnesia.  This  group  includes  the  felspars, 
which  derive  their  name  from  the  German  fels,  a  rock, 
and  spar,  a  crystalline  earthy  mineral,  and  were  so  named 
because  they  form  the  most  abundant  constituents  of 
igneous  rocks. 

Fourth,  the  acid  group  in  which  the  minerals  are  com- 
posed only  of  silica ;  the  only  common  member  of  this 
group  is  quartz. 

Most  igneous  rocks  consist  of  a  mixture  of  several  con- 
stituents, and  as  the  molten  material  solidifies,  species  of 
minerals  are  formed  which  may  belong  to  each  of  the 
four  groups.  The  molten  part,  therefore,  gradually 
changes  in  composition,  for  as  the  basic  materials  solidify 
first,  the  molten  residue  becomes  increasingly  acid.  Now, 
if  the  molten  mass  were  subject  to  heavy  pressure  before 
it  had  become  completely  solid,  the  still  molten  part 
would  be  squeezed  out  of  it.  The  rock  would  thus  be 
divided  into  two  parts — a  solid  part  composed  of  the 

81  F 


The  Materials  of  the   Earth's  Crust 

more  basic  materials,  and  a  liquid  part  consisting  mainly 
of  the  acid  materials.  The  squeezed-out  liquid  residue 
would  solidify  separately,  and  thus  the  original  rock 
magma  would  have  given  rise  to  two  different  rocks,  one 
of  which  is  basic  and  the  other  acid.  These  two  different 
rocks  would  have  been  produced  owing  to  their  con- 
stituents having  solidified  at  different  stages,  and  in 
consequence  of  a  "  differential  "  process.  The  two  rocks 
are  due  to  differentiation. 

The  great  chemist  Bunsen  pointed  out  in  1851  that  the 
igneous  rocks  of  the  earth  might  be  regarded  as  formed 
of  two  main  kinds,  with  various  intermediate  varieties. 
This  view  has  been  constantly  reasserted,  and  many 
different  names  have  been  proposed  for  the  two  main 
divisions ;  the  most  familiar  names  are  the  granitic  and 
basaltic.  Granite  and  basalt  are  two  of  the  most  wide- 
spread of  igneous  rocks,  and  the  further  action  of  the 
processes  which  have  made  their  differences  have  estab- 
lished beyond  them  a  few  still  more  extreme  modifications ; 
and  intermediate  between  granite  and  basalt  is  a  numerous 
series  of  stages,  such  as  might  be  produced  by  inter- 
mixtures of  various  proportions  of  the  two  typical  rocks. 

The  origin  of  these  varieties  has  been  attributed  to  two 
processes.  According  to  one  theory,  the  different  igneous 
rocks  are  due  to  "  differentiation  " — that  is  to  say,  to  the 
establishment  of  differences  in  an  originally  uniform  mass 
of  molten  rock  owing  to  its  constituents  being  irregularly 
sorted  during  solidification.  A  bottle  of  paste  may 
separate  into  a  liquid  layer  above  and  the  more  solid 
paste  below,  owing  to  the  water  being  gradually  separated 
from  the  solid  flour,  and  this  would  be  a  process  of 
differentiation.  Similarly,  it  is  held  that  in  a  mass  of 
molten  rock  the  heavier  constituents  will  tend  to  collect 
at  the  bottom,  and  the  lighter  on  top. 

This  process  of  differentiation  according  to  gravity 

82 


The  Materials  of  the  Earth's  Crust 

appears  to  have  acted  on  a  colossal  scale,  for,  as  was 
pointed  out  in  Chapter  III.,  the  separation  of  the  light 
materials  of  the  earth's  crust  from  the  metals  which  form 
its  central  mass  was  doubtless  due  to  the  lighter  materials 
floating  upward  like  a  slag,  while  the  heavy  metals  sank. 
Darwin  and  many  later  geologists  have  invoked  this 
process  to  explain  the  difference  between  adjacent  igneous 
rocks ;  but  some  authorities  insist  that  this  process  does 
not  take  place  in  rocks  to  any  important  extent.  It  is  true 
that  in  many  cases  the  lower  parts  of  a  great  mass  of 
igneous  rock  and  the  bottom  of  a  thick  lava  flow  do  not 
appear  to  be  heavier  than  the  rest.  Nevertheless,  the 


THE    IGNEOUS    SHEET 


FIG.  6. — DIAGRAMMATIC  LONGITUDINAL  SECTION  OF  A  SHEET  OF 
IGNEOUS  ROCK  AT  LUGAR,  AYRSHIRE  (TYRRELL). 

Not  drawn  to  scale.  Length  of  sill,  3^  miles ;  thickness  140  feet.  The 
sinking  of  the  heavier  materials  has  separated  the  main  mass  into  a 
lighter  upper  portion  (picrite)  and  a  heavier  lower  portion  (peridotite). 

process  appears  sometimes  to  happen.  Thus,  Mr.  Tyrrell 
has  shown  that  in  a  sheet  of  igneous  rock  at  Lugar,  in 
Ayrshire,  the  heavier  constituents  have  collected  on  the 
under  side,  so  that  the  upper  and  lower  parts  of  the  one 
sheet  of  rock  are  two  distinct  rock  types  (Fig.  6). 

The  unequal  distribution  of  the  heavier  materials  in 
molten  rock  has  also  been  attributed  to  changes  during 
cooling,  because  the  dissolved  matter  in  a  cooling  solution 
collects  in  the  coldest  parts  of  it.  Therefore,  as  a  mass  of 
molten  rock  cools  and  solidifies,  the  basic  materials  ought 
to  collect  on  the  cooling  margin  while  the  hotter  central 
part  remains  more  acid.  It  is,  however,  argued  that 

83 


The  Materials  of  the  Earth's  Crust 

though  this  process  would  operate  to  some  extent,  its 
influence  is  trivial. 

A  third  variety  of  differentiation  probably  happens  when 
rocks  solidify  in  an  area  which  is  being  disturbed  by  earth 
movements,  so  that  the  rocks  are  in  a  state  of  flow.  In 
such  a  case,  after  some  of  the  constituents  have  solidified, 
the  rest  of  the  liquid  rnay  be  squeezed  out,  as  juice  is 
squeezed  out  of  a  lemon.  Thus  some  very  acid  material 
may  be  forced  out  of  a  solidifying  rock,  which  is  thus  left 
more  basic  than  it  was  at  first. 

It  has  been  claimed  that  the  different  varieties  of  igneous 
rocks  have  been  developed  from  the  two  original  types  by 
the  partial  sorting  out  of  the  constituents  during  solidifi- 
cation. This  view  was  especially  predominant  in  geology 
from  about  1890  to  1905. 

In  recent  years  growing  importance  has  been  attached  to 
the  rival  process  of  the  formation  of  rocks  by  intermixtures 
or,  as  it  is  called,  by  "assimilation,"  since  one  rock  is  sup- 
posed to  vary  by  the  absorption  and  assimilation  of  alien 
material.  For  instance,  it  is  claimed  that  the  typical  lavas 
of  the  Pacific  area  have  absorbed  so  much  calcareous  matter 
that  they  are  unusually  rich  in  lime  and  magnesia;  while, 
according  to  Jensen,  the  lavas  of  the  Atlantic  area  owe 
their  richness  in  soda  and  potash  to  the  absorption  of 
marine  deposits. 

Prof.  Lowinsson-Lessing,  in  a  discussion  of  the  two 
hypotheses,  divides  igneous  rocks  into  those  which  have 
been  formed  from  the  two  primary  magmas,  granite  and 
basalt,  those  which  have  been  formed  from  these  rocks  by 
differentiation,  and  those  which  have  been  formed  by 
assimilation. 

That  igneous  rocks  can  absorb  and  assimilate  other 
rocks  was  early  accepted  by  many  mining  men ;  they 
recognized  that  sometimes  a  sheet  of  igneous  rock  shows 
no  signs  of  having  pushed  aside  the  neighbouring  beds,  but 


The  Materials  of  the  Earth's  Crust 

appears  to  have  eaten  its  way  along  one  particular  bed. 
This  bed  has,  in  fact,  been  replaced  by  the  igneous  rock 
and  not  displaced  by  it.     In  such  a  case,  the  igneous  rock 
must  have  slowly  advanced  and  have  eaten  and  completely 
digested  the  bed  that  has  disappeared.     The  igneous  rock 
would  have  been  thereby  necessarily  altered  in  compo- 
sition.    This  view  appeared  so  improbable  that  for  a  long 
time  geologists  were  generally  disposed  to  reject   it  as 
based   on  erroneous   observations.     Igneous  rocks   often 
have  an  extremely  sharp  margin,  as  if  they  had  cut  across 
the  rocks  beside  them  like  a  wedge  and  had  not  absorbed 
any  of  the  adjacent  material.     Nevertheless,  as  early  as  in 
1872  Lemberg  advocated  the  view  of  igneous  absorption, 
and  the  supposed  oldest  known  form  of  life,  Eozoon,  was 
explained  by  Dr.  Johnston-Lavis  and  the  author  in  1894 
as  due  to  assimilation  having  taken  place  between  some 
blocks  of  limestone  and  the  lava  which  enclosed  them. 
Michel- Levy  described  cases  of  the  intimate  intermixture 
of  thin  leaves  of  granite  between  layers  of  schist,  producing 
a  composite  rock.     If  a  bottle  of  ink  were  spilt  over  a  pile 
of  sheets  of  zinc,  the  ink  would  run  in  between  the  pieces 
of  zinc  and  the  pile  would  consist  of  alternate  sharply 
separated  layers  of  ink  and  metal ;  but  if  the  same  experi- 
ment were  tried  with  pieces  of  slowly  absorbent  card,  the 
ink  would  gradually  work  its  way  into  them  until  in  time 
instead  of  alternate  layers  of  ink  and  white  cardboard^ 
there  would  be  a  uniform  black  mass.    Similarly  the  leaves 
of  molten  granite  might  unite  with  the  layers  of  schist,  and 
thus  form  a  uniform  rock  showing  no  trace  of  its  compound 
origin. 

As  to  the  extent  to  which  this  assimilation  takes  place, 
there  has  been  a  long  controversy  mainly  between 
geologists  who  have  recognized  the  difficulties  suggested 
by  physical  chemistry,  and  those  who  have  insisted,  from 
the  facts  observed  in  the  field,  that  this  process  must  have 

85 


The  Materials  of  the  Earth's  Crust 

taken  place.  In  recent  years  the  view  that  many  rocks 
have  been  formed  by  assimilation  has  met  with  increasing 
acceptance.  Many  French  geologists  have  followed  Michel- 
Levy  ;  the  geologists  of  Scandinavia  and  Finland  have 
been  led  by  the  same  view  to  explain  the  characteristics 
of  their  Archean  rocks;  Prof.  Cole  in  Ireland  and  Prof. 
Daly  in  America  may  be  mentioned  as  leading  champions 
of  the  view  that  many  igneous  rocks  owe  their  special 
characters  to  the  absorption  of  sedimentary  material. 

Among  the  Archean  rocks,  which  form  the  lowest  seen 
foundation  of  the  earth's  crust,  all  stages  are  visible  from 
sedimentary  rocks  which  are  traversed  by  a  few  igneous 
veins,  through  a  network  of  igneous  veins  with  lumps 
of  sedimentary  rock  included  in  them,  to  igneous  rocks 
containing  small  barely  recognizable  fragments  of  sedi- 
mentary material.  This  transition  appears  to  indicate 
that  the  sedimentary  foundation  of  the  crust  has  been 
invaded  by  such  quantities  of  igneous  material  that  it  has 
been  sometimes  melted  up  and  has  thus  formed  composite 
igneous  rocks.  This  hypothesis  is  a  resurrection  of  the 
old  view  that  represented  the  rocks  of  the  earth's  crust  as 
passing  through  a  constant  cycle  of  change.  The  igneous 
rocks  are  broken  up  into  sedimentary  rocks,  such  as 
sandstone,  clay,  and  limestone,  and  these  have  been 
altered  into  metamorphic  rocks,  which,  by  further  change, 
have  passed  into  igneous  rocks ;  these  last  have  then  been 
broken  up  into  sediments,  and  the  cycle  restarted.  That 
sedimentary  rocks  have  been  converted  into  igneous  rocks 
has  probably  happened  occasionally ;  but  the  present 
tendency  seems  to  be  to  exaggerate  its  extent  as  much  as 
twenty  years  ago  its  importance  was  underrated. 

Many  of  our  oldest  Archean  rocks  in  different  parts  of 
the  world  retain  as  a  mass  all  the  essential  characteristics 
of  a  sedimentary  deposit.  Thus  the  Moine  Gneiss,  which 
is  the  largest  constituent  of  the  Scottish  Highlands,  is  still 

86 


The  Materials  of  the  Earth's  Crust 

recognizable  as  of  sedimentary  origin  in  spite  of  its 
antiquity,  of  the  extent  to  which  it  has  been  intruded  by 
igneous  rocks,  and  of  the  influence  of  the  great  earth 
movements  which  it  has  undergone.  In  other  parts  of  the 
earth's  crust  the  rocks  may  have  sunk  so  deeply,  or  may 
have  been  so  altered  by  ascending  hot  gases,  that  they 
have  been  fused,  and  the  signs  of  their  sedimentary  origin 
destroyed ;  yet  it  appears  to  be  still  possible  in  the  main 
to  separate  the  ancient  sedimentary  from  the  ancient 
igneous  rocks. 

Dr.  Harker,  who  is  one  of  the  leading  opponents  of  the 
assimilation  theory,  insists  that  there  is  no  adequate  source 
of  heat  for  such  remelting.  He  has  himself  described 
cases  of  such  "  hybrid  "  igneous  rocks,  but  he  insists  that 
their  characters  are  so  abnormal  that  they  form  no  excep- 
tion to  the  rule  that  assimilation  is  not  an  important 
factor  in  the  origin  of  rocks. 

Prof.  Suess,  however,  has  invoked  the  uprise  of  hot 
gases  from  the  interior  of  the  earth,  and  considers  that 
under  their  influence  some  of  the  deeper  parts  of  the  crust 
have  been  melted  into  igneous  rocks. 

The  more  general  acceptance  of  the  belief  that  the 
ancient  crust  has  been  in  places  altered  beyond  recognition 
owing  to  the  ascent  of  plutonic  gases  and  waters  indicates 
the  growing  recognition  of  the  important  influence  which 
the  internal  mass  of  the  globe  has  exercised  upon  its  cold 
outer  crust. 

A  full  and  lucid  statement  of  the  problems  connected  with  the 
origin  of  the  igneous  rocks  is  given  in  Harker's  "The  Natural  History 
of  Igneous  Rocks  "  (1909).  A  shorter  and  more  popular  account  of 
rocks  in  general  is  given  by  Cole,  "  Rocks  and  their  Origin  "  ("  Cam- 
bridge Manuals,"  1912). 


PART  II 
PHYSICAL  GEOLOGY 

CHAPTER  V 

THE  WEARING  DOWN  AND  UPLIFTING  OF  THE 
EARTH'S  CRUST 

PHYSICAL  geology  is  that  branch  of  the  science  which 
deals  with  the  action  of  the  various  geographical  agents 
that  affect  the  surface  of  the  earth.  It  includes  the  study 
of  the  geological  action  of  the  wind,  rivers  and  streams, 
sea,  ice,  earthquakes,  and  volcanoes.  The  geographical 
agents  have  been  the  same,  and  have  worked  apparently 
with  about  the  same  strength,  throughout  geological  time. 
For  in  some  of  the  earliest  times  of  which  there  are  clear 
records,  the  temperature  of  the  earth  was  much  the  same 
as  it  is  now,  and  the  winds  blew  with  their  present 
strength.  The  drops  of  rain  were  of  the  same  size,  and 
fell  with  the  same  force;  and  the  grains  of  sand  and 
pebbles  carried  down  by  the  most  ancient  rivers  were 
no  larger  than  those  moved  by  rivers  to-day.  Of  course 
there  have  been  local  variations  in  the  nature  and  strength 
of  these  forces.  In  some  localities  the  wind  is  much  more 
powerful  than  in  others ;  and  some  places  may  in  the  past 
have  been  swept  by  even  stronger  winds  than  any  which 
blow  on  the  earth  to-day.  Past  climatic  variations  have 
produced  great  local  changes,  but  the  world  as  a  whole 
appears  all  through  its  geological  history  to  have  had  a 
very  similar  climate  to  that  which  it  now  enjoys. 

88 


Wearing  Down  of  the  Earth's  Crust 

The  present  form  of  the  earth's  surface  is  the  result 
of  two  groups  of  opposing  forces.  The  members  of  one 
group  tend  to  destroy  the  rocks,  wear  away  the  land,  and 
sweep  the  materials  into  the  sea.  These  processes  are 
opposed  by  the  forces  of  the  second  group,  which  tend  to 
raise  the  continents  and  lower  the  sea-floors,  so  that  the 
land  is  kept  divided  from  the  waters ;  and  both  are 
maintained  as  the  homes  of  separate  kinds  of  animals 
and  plants. 

Owing  to  the  combined  action  of  the  various  geo- 
graphical agencies,  the  surface  of  the  land  is  of  extreme 
irregularity.  The  land  areas  differ  greatly  both  in  plan 
and  in  relief.  It  is  possible  to  conceive  a  world  in  which 
land  and  water  might  be  distributed  with  the  regularity  of 
a  chess-board.  The  land  might  occur  in  low  square 
sections,  and  the  seas  might  occupy  basins  all  bounded 
by  straight  lines  and  of  uniform  depth.  Such  a  world 
would  be  so  different  from  our  own  that  no  such  civiliza- 
tion as  ours  could  have  been  developed  on  it ;  it  would, 
indeed,  be  hardly  habitable  by  the  higher  forms  of  life. 
But  fortunately  the  earth  is  divided  into  areas  which  are 
so  irregular  that  they  supply  the  diversity  of  geographical 
conditions  necessary  for  our  complex  wants  and  continuous 
development. 

It  is  true  that  both  continents  and  oceans  have  a 
remarkable  underlying  community  in  shape  and  arrange- 
ment; but  in  detail  they  are  very  different,  the  land 
rising  and  falling  in  an  intricate  series  of  hills  and  valleys, 
highlands  and  lowlands.  The  differences  are  so  well 
marked  that  the  maps  of  various  continents  and  countries 
are  so  varied  that  they  can  be  distinguished  at  a  glance 
without  reference  to  the  place-names. 

The  sea  is  divided  from  the  land  by  a  sinuous  shore- 
line ;  the  land  projects  into  the  sea  where  the  hills  reach 
the  shore,  and  the  sea  runs  into  the  land  as  great  sea 


The  Wearing  Down  and 

valleys,  or  floods  the  continuations  of  the  lowlands  as 
inland  seas.  These  irregularities  of  the  shore-line  are  due 
to  elevations  and  depressions  of  the  earth's  crust  which 
give  rise  to  five  land-forms — plains,  mountains,  and 
plateaus  constituting  the  raised  areas,  valleys  and  basins, 
the  two  negative  land-forms,  forming  the  depressions. 
The  arrangement  of  the  land-forms  determines  the  shape 
both  of  land  and  sea.  The  greater  land-forms  which  form 
the  continents  and  oceans  are  due  to  movements  of  the 
crust.  The  plateaus  are  due  to  widespread  uplifts,  and  to 
areas  which  have  been  left  upraised  owing  to  the  sinking 
of  the  land  around  them.  The  great  mountain-chains 
have  been  formed  by  the  crumpling  of  long  narrow  bands, 
the  ocean  and  sea  basins  and  some  of  the  widest 
plains  occupy  areas  that  have  sunk  below  the  general 
level. 

The  minor  irregularities  on  the  earth's  surface  are  due 
to  the  agents  which  carve  the  surface  of  the  land  into 
valleys  and  basins ;  they  are  chiefly  the  wind,  rain,  rivers, 
glaciers,  and  sea.  The  work  of  these  excavating  agencies 
is  known  as  "denudation."  The  term  means  "laying  bare," 
and  it  is  used  for  all  those  processes  by  which  the  surface 
of  the  earth  is  worn  away  and  new  layers  are  exposed,  to 
be  removed  in  their  turn. 


PROCESSES  OF  DENUDATION. 

The  rocks  exposed  on  the  surface  of  the  earth  slowly 
decay  and  crumble  in  consequence  of  the  action  of  the 
atmosphere;  for  two  of  the  gases  in  the  air  unite  with 
some  constituents  in  the  rocks,  and  this  union  is  accom- 
panied byan  expansion,  which  helps  the  rocks  to  crumble 
away.  The  rain  aids  the  process  by  dissolving  any  soluble 
material  in  the  rock,  and  thus  allowing  the  air  to  pene- 
trate more  freely.  Frost  helps  greatly  by  freezing  water 

90 


Uplifting  of  the  Earth's  Crust 

that  has  soaked  into  the  pores  of  a  rock,  for  the  water 
expands  suddenly  as  it  is  turned  into  ice,  and  helps  to 
tear  the  rock  to  pieces.  The  change  of  temperature  be- 
tween night  and  day  also  tends  to  break  up  the  rocks 
owing  to  their  expansion  when  heated  and  their  contrac- 
tion on  cooling.  Many  rocks  are  composed  of  several 
constituents,  which  expand  and  contract  at  different  rates, 
and  such  rocks  are  rent  by  innumerable  small  cracks 
during  changes  of  temperature. 

The  destructive  effect  of  these  various  forces  is  shown 
by  the  rapid  decay  of  building-stones,  which,  if  badly 
selected,  may  last  a  shorter  time  than  timber.  Even  care- 
fully selected  building-stone  decays  in  a  city  atmosphere. 
Thus,  although  tombstones  are  made  of  specially  selected 
slabs,  it  is  rare  to  find  on  them  legible  inscriptions  more 
than  a  couple  of  centuries  old.  During  that  time  they 
have  lost  a  surface  layer  as  thick  as  the  depth  of  the 
inscription. 

Many  building-stones,  especially  limestones,  decay  with 
deplorable  rapidity  in  the  atmosphere  of  great  cities.  The 
stone  of  the  Houses  of  Parliament  at  Westminster  has 
crumbled  so  rapidly  that  many  of  the  stone  ornaments 
upon  the  building  have  been  replaced  by  cast  iron,  and 
the  outside  of  Westminster  Abbey  is  said  to  have  been 
renewed  five  times  over  ;  nor  is  the  atmosphere  of  London 
by  any  means  exceptionally  destructive.  I  recently 
searched  in  vain  on  the  great  western  front  of  Amiens 
Cathedral  for  any  of  the  original  stone  placed  there  by  the 
builders  in  the  fifteenth  century. 

The  destructive  effect  of  moisture  and  gases  in  the  air 
is  aided  by  the  wind ;  the  loose  dust  on  a  city  street 
includes  particles  of  rock,  fragments  of  iron  from  horses' 
shoes  and  wheel  tyres,  and  splinters  of  other  hard 
materials.  These  are  being  flung  continually  by  the  wind 
against  projecting  surfaces,  which  they  slowly  wear  away. 


The  Wearing  Down  and 

Even  soft  materials,  if  thrown  against  a  hard  body  by 
a  strong  blast  of  air,  will  gradually  cut  into  it ;  ordinary 
wheat-flour  thrown  by  a  powerful  jet  of  air  against  a  sur- 
face of  rock  crystal  will  eat  into  it.  This  is  the  principle 
of  the  sand-blast,  when  a  jet  of  sand  is  used  to  polish  and 
•  etch  glass.  The  wind  acts  like  a  powerful  natural  sand- 
blast ;  it  undercuts  cliffs,  wears  away  projecting  rocks, 
and  may  in  time  level  a  line  of  hills.  The  materials  worn 
by  the  wind  from  the  land  at  one  place  are  carried  forward 
and  deposited  in  sheltered  hollows,  where  they  may  collect 
as  a  thick  bed  of  subaerial  deposits  ;  and  in  this  Way  the 
wind  blowing  over  a  wide  undulating  country  may  level 
the  surface  by  lowering  the  hills  and  filling  up  the 
hollows. 

Water  is  by  far  the  most  powerful  of  all  geographical 
agents.  In  denudation  it  acts  mainly  in  streams  and 
rivers.  Rivers  file  away  their  beds  by  means  of  the  sand 
and  stones  which  they  roll  along  ;  and  the  long-continued 
wearing  downward  of  the  river-bed  deepens  valleys  into 
gorges  or  canyons,  of  which  the  walls  are  steep  and  high 
in  proportion  to  their  width.  The  rate  at  which  a  river 
-deepens  its  channel  is  greatly  quickened  by  the  formation 
of  pot-holes.  A  boulder  lying  on  a  river-bed  is  spun 
around  by  the  current,  and  thus  wears  a  small  hole  below 
it.  The  hole  once  started  tends  to  increase  in  depth,  as 
sand  and  pebbles  are  washed  into  it,  and  kept  swirling 
there  by  an  eddy.  Neighbouring  pot-holes  are  enlarged 
and  at  last  joined,  and  the  ridges  and  pinnacles  of  rock 
left  between  them  are  broken  down,  and  thus  bars  of  hard 
rock  are  in  time  gradually  worn  away, 

Rivers,  therefore,  by  continually  filing  and  boring  into 
their  beds,  first  excavate  deep,  narrow  canyons.  Then 
wind,  rain,  and  frost,  gradually  wear  back  the  walls  into 
sloping  banks,  and  the  widening  process  is  often  aided 
by  land-slips.  The  fallen  material  is  swept  away  by  the 

92 


Uplifting  of  the  Earth's  Crust 

torrent,  and  the  gorge  is  gradually  converted  into  a  wide 
valley.  The  river  in  time  wears  its  bed  into  a  long,  even 
descent  from  its  source  to  the  sea,  while  wind  and  rain 
cut  the  banks  on  either  side  into  clifHess  slopes.  Ulti- 
mately the  valley  of  an  ancient  river  has  gently  sloping 
banks,  and  its  lower  part  is  usually  occupied  by  level 
plains,  with  a  gradual  incline  downward  to  the  sea. 

The  destruction  of  the  land  is  also  helped  by  the  sea. 
The  surf  batters  the  cliffs,  the  tidal  currents  sweep  away 
beach  material,  and  may  thus  keep  new  rock  surfaces 
always  exposed  to  attack.  The  cutting  back  of  the  land 
by  coast  erosion  is  often  so  rapid  that  whole  parishes  have 
disappeared  from  the  eastern  coast  of  England,  and  a 
prebendal  stall  in  St.  Paul's  Cathedral  is  said  to  have  lost 
its  endowment  owing  to  the  land  which  paid  it  now  lying 
beneath  the  North  Sea  off  the  Essex  coast. 

In  some  parts  of  the  world,  especially  in  snow-covered 
mountains  and  in  the  Arctic  and  Antarctic  regions,  ice 
also  plays  an  important  part  in  wearing  down  the  land. 
If  more  snow  falls  on  the  mountains  in  winter  than  is 
melted  during  the  summer,  the  excess,  converted  into  ice, 
flows  down  into  the  valleys  as  a  glacier. 

A  glacier  is  a  river  of  ice,  which  flows  down  its  valley 
like  a  river  of  water,  but  with  far  less  speed.  It  carries 
with  it  material  that  has  fallen  on  to  the  glacier  from 
cliffs  or  that  has  been  frozen  to  the  under  side  of  the 
glacier,  and  the  dirt  and  stones  rubbing  against  the  rocks 
beneath  the  glaciers  must  in  time  help  to  wear  them 
away. 


93 


CHAPTER  VI 
FOLDS  AND   FAULTS 

MANY  geographical  processes  are  therefore  constantly 
wearing  down  the  surface  of  the  earth.  The  action  of 
rain  and  rivers  would  in  time  lower  all  the  land  to  a 
smooth  slope,  rising  from  the  coast  to  the  interior  at  the 
rate  of  about  a  foot  in  ten  miles.  Water  action  cannot 
reduce  the  slope  further,  for  that  is  the  most  gradual 
gradient  down  which  water  can  carry  sediment.  The 
wind,  however,  could  continue  to  work,  and  by  sand 
erosion  plane  the  whole  land  down  to  sea-level.  Other 
agencies,  moreover,  cause  the  land  to  sink  in  mass, 
Owing  to  the  cooling  of  the  earth,  its  mass  shrinks,  and 
the  surface  subsides  in  area  after  area;  and  volcanic 
eruptions  leave  the  ground  unsupported,  and  wide  tracts 
of  land  may  founder,  owing  to  the  collapse  of  subterranean 
hollows,  and  may  thus  sink  beneath  the  sea.  The  ulti- 
mate annihilation  of  the  land  is  only  prevented  by  agencies 
which  uplift  it,  so  that  land  has  existed  throughout  the 
whole  of  geological  time,  in  spite  of  the  forces  which  are 
continually  destroying  it. 

The  uplift  of  the  land  may  be  slow  and  imperceptible, 
or  may  take  place  by  sudden  jerks.  The  gentle  rise  and 
fall  of  the  ground  is  well  illustrated  by  the  story  of  the 
Temple  of  Jupiter  Serapis  told  in  Chapter  I.  The  move- 
ment there  affects  only  a  small  area,  for  Naples,  five  miles 
to  the  east,  has  not  shared  this  oscillation.  Similar  move- 
ments take  place  on  a  continental  scale,  for  it  has  been 

94 


Folds  and  Faults 

proved,  after  long  controversy,  that  northern  Scandinavia 
is  steadily  rising,  so  that  old  shore-marks  are  now  above 
sea-level,  and  the  sea  has  receded  from  the  land.  Never- 
theless, southern  Sweden  and  the  northern  coast  of 
Germany  have  maintained  their  old  level  in  reference 
to  the  Baltic  ;  so  that  the  uprise  of  the  land  to  the  north 
is  due  to  the  whole  of  Scandinavia  being  slowly  tilted. 
There  is  also  evidence  that  at  a  slightly  earlier  time  the 
British  Isles  were  tilted  by  an  uplift  to  the  north.  The 
remains  of  a  plateau  in  Scotland  show  that  the  land 
formerly  stood  a  thousand  feet  lower  than  at  present, 
while  the  corresponding  uplift  in  the  south  of  England 
was  only  about  half  that  amount.  In  later  times  Scot- 


•m 


FIG.  7. — FOLDS. 

m,  Monocline  ;  s,  symmetrical  syncline  ;  a,  symmetrical  anticline ;  u.a.,  un- 
symmetrical  anticline ;  u.s.,  unsymmetrical  syncline ;  i,  isoclines. 

land  was  raised  one  hundred  feet,  while  the  south-east 
of  England  was  stationary  ;  for  the  raised  beaches  which 
are  so  conspicuous  on  the  Scottish  coasts  show  an  uprise 
of  the  land  with  rests  at  twenty-five,  fifty,  and  a  hundred 
feet  above  the  present  sea-level,  while  south-eastern 
England  during  the  same  period  was  either  stationary 
or  perhaps  sinking. 

The  movements  of  the  crust  are  due  to  two  different 
processes  —  folding  and  faulting.  If  a  tablecloth  be 
pushed  across  a  table,  it  is  thrown  into  a  series  of 
ridges  and  valleys  like  a  sheet  of  corrugated  iron ; 
and  if  a  part  of  the  earth's  crust  is  squeezed  into 

95 


Folds  and  Faults 


a  smaller  space,  it  is  likewise  buckled  into  a  series 
of  folds.  They  are  known  as  anticlines  (Fig.  7,  a)  or 
upfolds,  when  the  beds  are  bent  upward  as  in  an  arch ; 
as  synclines  (s)  or  downfolds,  when  the  beds  sink  into 
a  trough  ;  as  monoclines  (m)  when  the  fold  has  only  a 
single  slope,  joining  the  ground  raised  from  its  continua- 


ft     /=•    f 


FIG.  8.— -FAULTS. 

a,  Normal  fault ;  b,  reversed  fault ;  c,  trough  fault ;  d,  ridge  fault ; 
e,  three  step  faults ;  /,  thrust-plane  ;  F,  fault ;  Tt  thrust. 

tion  on  a  lower  level ;  as  isoclines  (i)  when  the  folds 
have  been  so  pressed  together  that  the  summits  have  been 
forced  over  so  that  both  sides  of  the  fold  slope  in  the  same 
direction.  Isoclines  occur  where  mountains  have  been 
formed  by  intense  compression  of  plastic  rocks.  Gentle 
upfolds  and  downfolds  may  be  due  merely  to  the  slight 


Folds  and   Faults 

displacement  of  the  isolated  rigid  blocks  by  the  opening 
of  cracks  between  them,  just  as  a  wooden  pavement  may 
be  bent  by  the  gaping  of  its  joints.  In  such  movements 
adjacent  blocks  may  be  displaced  in  level ;  one  block  may 
rise  or  fall  along  the  joint  between  it  and  its  neighbour  ; 
and  such  displacements  along  straight  fractures  are  known 
as  faults.  In  a  normal  fault  (Fig.  8,  a)  the  plane  of  move- 
ment slopes  down  toward  the  lower  side,  as  the  two  sides 
slipped  apart.  In  a  reversed  fault  (b)  the  two  sides  are 
squeezed  together  so  that  the  side  which  is  lowered  is 
forced  under  the  upper  side. 

Two  parallel  faults  sloping  in  opposite  directions 
beside  a  strip  of  sunken  land  form  a  trough  fault  (c).  A 
pair  of  parallel  faults,  leaving  a  ridge  between  them,  form 
a  ridge  fault  (d).  A  series  of  faults  with  the  slope  and 
the  direction  of  the  movement  all  in  the  same  direction 
is  a  series  of  step  faults  (e).  A  reversed  fault,  in  which 
the  movement  sideways  is  greater  than  the  vertical  move- 
ment, is  a  thrust  plane  (/). 

The  movements  of  the  earth  crust  during  folding  and 
faulting  are  felt  as  earthquakes.  The  movements  cause 
unequal  pressure  on  the  deeper  parts  of  the  crust,  which 
is  often  relieved  by  volcanic  eruptions ;  and  they  crumple 
the  crust  into  mountain-chains. 


97 


CHAPTER    VII 

EARTHQUAKES 

IF  you  watch  a  big  wheel  revolving  rapidly,  it  may,  if  well 
constructed,  appear  at  first  sight  to  be  motionless ;  but  if 
you  observe  it  more  closely,  you  will  probably  see  the  wheel 
vibrating  and  trembling,  and  occasionally  it  may  quiver, 
as  if  it  had  been  slightly  shaken.  These  slight  tremors 
are  due  to  various  causes ;  the  wheel  may  not  have  been 
mounted  with  perfect  accuracy  on  its  axle,  or  the  spokes 
and  different  parts  of  the  rim  may  vary  slightly  in  weight, 
and  the  stronger  tremors  may  be  caused  by  slight  changes 
in  the  speed  of  rotation.  The  earth  is  revolving  like  a 
huge  flywheel,  and  its  crust  is  constantly  trembling  in 
consequence  of  many  slight  disturbing  influences.  The 
weight  is  not  evenly  distributed,  and  the  distribution  is 
being  constantly  altered,  for  the  tides  are  always  shifting 
huge  weights  of  water  from  one  part  to  another ;  and  it 
has  recently  been  proved  that  the  attraction  of  the  sun 
and  moon,  which  causes  the  tidal  movements  in  the  ocean, 
also  causes  a  tidal  rise  and  fall  in  the  solid  crust.  More- 
over, falls  of  rain  and  snow  place  a  heavy  additional 
weight  upon  one  area,  and  thus  disturb  the  previous 
balance  of  the  crust.  The  amount  of  water  that  falls  in  a 
heavy  storm  is  of  enormous  weight.  Thus  on  August  25 
to  26,  1912,  the  weight  of  water  which  fell  on  the  county 
of  Norfolk  (area  2,044  square  miles)  is  estimated  by  Dr. 
H.  R.  Mill  at  670,720,000  tons.  That  the  addition  of 
such  a  load  causes  the  crust  to  sink  is  not  mere  conjecture, 


Earthquakes 


for  the  late  Prof.  Milne  recognized  this  by  the  tilting  of 
his  instruments  at  Tokio  in  eastern  Japan  when  there  had 
been  heavy  storms  of  rain  on  the  western  side  of  Japan. 
It  is,  moreover,  now  well  established  that  the  axis  of  the 
earth  is  not  absolutely  constant,  so  that  the  North  Pole 
wanders  over  a  space  of  twenty  yards  or  more  in  diameter. 
This  shifting  of  the  Pole  shows  that  the  earth  wobbles 
during  its  spin  like  a  pegtop  that  is  not  quite  true ;  and 
this  wobbling  of  the  earth  has  been  attributed  to  the 
weight  of  heavy  falls  of  rain  or  snow  on  one  part  of  the 
North  Polar  regions. 

The  extreme  sensitiveness  of  the  earth's  crust  is  shown 
by  the  fact  that  many  earthquakes  are  produced  by  varia- 
tions in  atmospheric  pressure,  for  Japanese  statistics  show 
that  earthquakes  are  more  frequent  in  winter  than  in 
summer,  as  in  winter  the  changes  in  the  pressure  of  the  » 
atmosphere  are  more  sudden. 

These  tidal  and  meteorological  influences,  though  com- 
paratively slight,  are  always  at  work,  and  they  keep  the 
crust  of  the  earth  in  continual  vibration.  The  small 
movements  are  known  as  "  earth  tremors,"  and  they  can 
only  be  recognized  by  very  delicate  instruments ;  but  from 
these  slight  tremors  there  is  a  gradual  passage  to  shocks 
with  sufficient  power  to  shake  the  whole  earth  and  dev- 
astate a  province.  These  greater  disturbances  of  the 
earth's  crust  are  known  as  "  earthquakes  " ;  but  they  are 
essentially  of  the  same  nature  as  the  slight  tremors. 

An  earthquake,  however  caused,  is  a  wavelike  move- 
ment in  the  crust  of  the  earth.  A  water-wave  produces 
only  an  oscillation  in  the  particles  it  affects,  without  per- 
manently displacing  them  forward  or  backward.  When 
the  wave  has  passed,  the  particles  usually  occupy  the  same 
place  as  they  did  before  the  wave  affected  them.  The 
return  of  the  particles  to  their  original  position  may  be 
recognized  in  a  waving  field  of  corn.  When  the  wind 

99 


Earthquakes 


blows  across  a  cornfield,  the  corn  appears  to  travel  for- 
ward across  the  field ;  but  the  ears  are  anchored  to  the 
ground  by  the  stalks,  so  the  forward  movement  is  clearly 
deceptive.  When  closely  watched,  each  ear  may  be  seen 
to  rise  and  fall,  and  move  a  little  forward  and  backward ; 
but  it  returns  after  the  wave  has  passed  to  its  original 
position.  A  sea-wave  causes  the  particles  of  water  to 
move  on  a  circular  or  oval  path,  and  does  not  carry  them 
permanently  forward.  When,  however,  the  waves  strike 
a  sloping  beach,  the  normal  wave  movement  is  upset  by 
friction  with  the  ground,  and  the  water  is  flung  forward 
and  backward  in  mass. 

A  wave  in  which  the  distance  from  one  crest  to  the 
next  is  very  great  in  proportion  to  the  height  between 
the  top  of  the  crest  and  the  bottom  of  the  trough  between 
two  crests  is  said  to  be  a  long  wave.  Such  long  waves  may 
traverse  the  surface  of  the  earth  and  be  quite  inappreciable. 
Thus  the  earthquake  due  to  the  eruption  of  Mont  Pelee 
in  the  West  Indies  in  1902  sent  a  wave  through  the  earth 
which  caused  the  ground  at  Melbourne  to  rise  and  fall  for 
nine  hours  ;  but  as  the  waves  there  were  very  long  and 
very  low,  the  effect  was  not  felt  by  the  inhabitants,  though 
it  was  recorded  by  the  instruments  at  the  observatory. 
But  in  the  immediate  neighbourhood  of  the  place  where 
an  earthquake  is  started  movable  objects  may  be  thrown 
forward;  the  ground  may  be  permanently  uplifted  or 
lowered;  the  surface  may  be  rent  by  fissures  or  drilled 
into  earthquake  craters ;  and  the  vibration  of  the  crust 
may  be  so  violent  that  all  buildings  may  be  shattered  and 
overthrown. 

The  vibration  of  the  surface  is  so  rapid  that  it  often 
produces  a  noise  known  as  the  Earthquake  Sound.  When 
the  ground  is  vibrating  vertically,  it  beats  the  air  like  the 
head  of  a  sounding  drum ;  and  the  noise  thus  made  is 
often  heard,  and  in  some  districts  where  there  are  no 

100 


A  ROAD  NEAR  WRIGHT,  CALIFORNIA 

It  was  buckled  and  fissured  by  the  San  Francisco  earthquake,  1906. 


THE  WRECKED  LIBRARY  OF  THE  STANFORD  UNIVERSITY,  SAN  FRANCISCO 


Earthquakes 


buildings  to  shake  or  trees  to  rustle,  the  earthquake  sound 
is  many  a  time  the  only  indication  of  an  earthquake.  The 
booming  noises  known  in  India  as  "the  Barisal  guns"  and 
in  Australia  as  "  the  bulldag "  or  "  desert  sound,"  are 
doubtless  due  to  earthquakes.  In  most  earthquakes,  how- 
ever, the  sound  made  by  the  vibration  of  the  ground  is 
drowned  in  the  roar  of  shaking  buildings,  the  rattling 
of  roofs,  and  the  rustle  of  trees  and  shrubs. 

Earthquakes  are  due  to  sudden  movements  and  slips 
in  the  earth's  crust ;  they  are  therefore  said  to  be  tectonic 
in  origin.  Some  are  due  to  volcanic  explosions,  but  many 
of  those  which  occur  in  volcanic  areas  are  not  volcanic 
in  origin.  Japan  is  one  of  the  most  earthquake-shaken 
districts  in  the  world,  and  it  has  many  volcanic  moun- 
tains ;  it  might  therefore  be  regarded  as  supporting  the 
connection  of  earthquakes  and  volcanoes.  But  it  really 
shows  that  they  are  distinct  in  nature,  for  the  most  powerful 
Japanese  earthquakes  are  not  in  the  volcanic  districts. 
Similarly  the  Andes  are  close  to  a  great  earthquake  belt, 
and  include  famous  volcanoes ;  but  the  earthquakes  are 
as  frequent  in  the  non-volcanic  as  in  the  volcanic  parts 
of  the  Andes.  Southern  Italy  contains  the  only  active 
volcano  on  the  mainland  of  Europe ;  Sicily  has  in  Etna 
the  greatest  of  the  present  European  volcanoes,  and  the 
adjacent  region  in  both  places  has  been  the  scene  of  the 
most  catastrophic  earthquakes  in  European  annals. 

Cowper,  in  his  account  of  the  Messina  earthquake  of 
1783  ("The  Task,"  bk.  ii.,  published  1784),  expressed  the 
then  current  view  that  earthquakes  and  volcanoes  are  due 
to  a  common  cause : 

"  Alas  for  Sicily  !  rude  fragments  now 
Lie  scatter'd  where  the  shapely  column  stood. 
Her  palaces  are  dust.  .  .  . 

She  quakes  at  His  approach.     Her  hollow  womb 
Conceiving  thunders,  through  a  thousand  deeps 
And  fiery  caverns,  roars  beneath  His  foot. 
IOI 


Earthquakes 


The  hills  move  lightly,  and  the  mountains  smoke, 
For  He  has  touch'd  them.  .  .  . 
The  rocks  fall  headlong,  and  the  valleys  rise,  .  . 
What  solid  was,  by  transformation  strange, 
Grows  fluid,  and  the  fix'd  and  root'd  earth, 
Tormented  into  billows,  heaves  and  swells, 
Or,  with  vortiginous  and  hideous  whirl, 
Sucks  down  its  prey  insatiable.  .  .  . 
Ocean  has  caught  the  frenzy,  and  .  .  . 

invades  the  shore 

Resistless.  .  .  .    Where  now  the  throng 
That  press'd  the  beach,  and,  hasty  to  depart, 
Look'd  to  the  sea  for  safety  ?    They  are  gone, 
Gone  with  the  refluent  wave  into  the  deep — 
A  prince  with  half  his  people.     Ancient  towers, 
And  roofs  embattled  high,  .  .  . 
Fall  prone." 

Nevertheless,  the  greatest  of  the  Italian  earthquakes 
are  not  connected  with  the  volcanoes,  but  with  the  earth 
fractures.  The  great  Calabrian  earthquake  of  1783  did 
not  affect  the  areas  of  volcanic  rocks ;  the  terrible  Messina 
earthquake  of  1908,  though  it  devastated  cities  near  the 
eastern  foot  of  Etna,  arose  from  movements  in  the  Strait 
of  Messina ;  it  was  in  no  way  connected  with  Etna,  which 
was  not  even  disturbed  by  it. 

The  distribution  of  earthquakes  gives  the  most  con- 
clusive evidence  as  to  their  cause.  The  frequent  coin- 
cidence of  earthquakes  and  volcanoes  gave  rise  to  the  view 
that  they  are  intimately  associated.  Some  earthquakes 
are  of  volcanic  origin,  for  they  are  the  shocks  of  violent 
volcanic  explosions,  such  as  that  which  blew  the  mountain 
of  Krakatoa  to  fragments  in  1883,  and  that  which  formed 
the  deep  volcanic  rift  at  Tarawera  in  1884. 

Other  earthquakes  are  secondary  results  of  volcanic 
action ;  thus  in  July,  1883,  the  island  of  Ischia  was  devas- 
tated by  a  sudden  earthquake,  which  was  so  violent  in  the 
area  that  the  town  of  Casamicciola  was  overthrown  ;  only 
one  house  was  left  standing,  and  2,313  of  the  inhabitants 

102 


Earthquakes 


were  slain ;  but  this  shock,  violent  though  it  was  locally, 
was  so  narrow  in  its  range  that  it  was  only  felt  as  a  faint 


tremor  by  a  few  observers  in  Naples  twenty  miles  to  the 
east;  it  passed  unnoticed  at  Castello  d'Ischia  four  miles 

103 


Earthquakes 


from  the  centre,  and  was  not  recorded  by  the  sensitive  re- 
cording instruments  in  the  observatory  on  Vesuvius  less 
than  thirty  miles  distant.  (See  Map,  p.  117.) 

This  Ischian  earthquake  was  carefully  investigated  by 
Dr.  H.  J.  Johnston-Lavis,  and  he  concluded  that  it  was 
due  to  a  settlement  of  the  ground  about  2,000  feet  deep, 
owing  to  the  collapse  of  internal  cavities  left  vacant  by 
eruptions  of  the  volcano,  Monte  Epomeo. 

The  world-wide  observations  made  with  the  delicate 
recording  instruments  designed  by  the  late  Prof.  Milne 
for  a  Committee  of  the  British  Association  show  that  the 
chief  earthquake  centres  are  not  connected  with  volcanic 
action,  but  follow  the  chief  lines  of  earth  movements  in 
the  last  great  period  of  mountain  building. 

They  are  shown  in  the  accompanying  map  of  the  earth- 
quake regions,  and  on  it  are  placed  for  comparison  the 
chief  volcanoes.  i 

The  largest  earthquake  region  extends  along  the  line  of 
the  Alpine-Himalayan  Mountain  System  from  the  Alps  to 
central  China;  it  includes  Italy  and  the  eastern  Medi- 
terranean, the  sunken  basin  of  the  Caspian,  and  the 
young  and  probably  still  growing  mountain-chains  of  the 
Himalaya.  One -fifth  of  the  world-shaking  earthquakes 
originate  in  this  region.  Farther  east  is  the  great  earth- 
quake region  of  Malaysia,  which  extends  from  the  Bay  of 
Bengal  to  north  of  New  Zealand.  Its  axis  is  no  doubt 
the  young  mountain-line  which  traverses  the  Malay  Archi- 
pelago. To  the  north  of  this  belt  is  the  earthquake  region 
that  follows  the  eastern  coast  of  Asia  from  Kamschatka 
to  the  Philippine  Islands,  and  includes  the  archipelago  of 
Japan,  which  is  almost  constantly  shaken  by  earthquakes. 
On  the  opposite  side  of  the  Pacific  three  earthquake 
regions  occur  along  the  western  coast  of  America ;  one 
lies  along  the  coast  of  Alaska  and  British  Columbia ;  the 
next  begins  in  California,  includes  all  Central  America 

104 


Earthquakes 

and  ends  in  Ecuador ;  the  third  follows  the  coast  of  South 
America  from  Peru  to  Patagonia.  The  West  Indian  area, 
which  has  all  the  characteristics  of  great  structural 
instability,  is  also  a  region  of  unusually  active  earth- 
quakes; they  are  probably  due  to  the  earth  movements 
by  which  the  old  continent  of  Antillia  has  been  broken  up 
into  the  present  archipelago. 

The  site  of  a  more  completely  vanished  land  under 
the  Indian  Ocean  may  be  marked  by  the  earthquake 
region  which  extends  from  South  India  south-westward  to 
Mauritius  and  to  the  east  of  Madagascar.  The  founderings 
which  have  made  the  North  Atlantic  are  similarly  marked 
by  three  earthquake  regions ;  of  these  three,  one  lies 
north-east  of  Iceland,  and  is  parallel  to  the  coast  of 
Scandinavia,  and  extends  over  the  site  of  the  sunken 
basin  of  the  Norwegian  Sea ;  the  second  of  these  regions 
is  larger,  and  trends  from  North  Africa  northward  past 
Spain  and  Portugal,  and  off  the  Bay  of  Biscay  to  the 
west  of  Ireland ;  the  third  is  smaller  and  less  active,  and 
is  parallel  to  the  eastern  coast  of  the  United  States  and 
includes  the  islands  of  Bermudas. 

Some  of  these  earthquake  regions,  such  as  the  last  in 
the  list,  are  exceptionally  free  from  volcanic  activity ;  and 
even  in  a  region  containing  an  active  volcano,  the  most 
powerful  earthquakes  often  affect  the  non-volcanic  districts. 
Thus  the  appalling  Calabrian  earthquake  of  1783,  though 
near  the  volcanic  areas  of  Sicily  and  Vesuvius,  affected  a 
part  of  southern  Italy  where  there  are  no  volcanic  rocks. 
The  famous  Lisbon  earthquake  was  also  in  a  non-volcanic 
area.  As  a  general  rule,  though  earthquakes  often  happen 
in  volcanic  areas,  the  most  disastrous  and  most  powerful 
earthquakes  occur  in  non-volcanic  regions.  The  frequent 
coincidence  in  the  distribution  of  earthquakes  and  volcanoes 
is  due  to  their  dependence  on  a  common  cause — the 
collapse  of  large  areas  of  the  earth's  crust. 

105 


Earthquakes 


The  Assam  earthquake  of  I2th  June,  1897,  is  perhaps  the 
greatest  earthquake  which  has  been  carefully  investigated. 
It  is  fortunately  unique  in  the  wide  area  of  country  that 
was  devastated  by  it.  Its  effect  was  so  violent  over  so 
large  an  area,  that,  had  it  happened  under  central  England, 
not  a  building  or  bridge  between  Liverpool  and  London 
would  have  escaped  destruction. 

The  widespread  nature  of  this  violent  disturbance  might 
appear  inconsistent  with  the  view  that  earthquakes 
originate  at  shallow  depths  (see  p.  57) ;  but  Dr.  R.  D. 
Oldham,  who  investigated  this  earthquake  on  behalf  of 
the  Indian  Government,  concluded  that  it  was  caused 
at  a  depth  of  only  about  five  miles ;  and  its  widespread 
devastation  was  due  to  a  vast  area  at  that  depth  having 
been  moved  southward,  so  that  all  the  ground  above  this 
moving  mass  was  violently  disturbed. 

The  area  most  disastrously  affected  is  somewhat  tri- 
angular in  shape,  and  is  from  7,000  to  10,000  square  miles 
in  extent.  It  is  situated  to  the  north-east  of  Calcutta 
around  the  Khasi  Hills ;  the  northern  corner  just  crosses 
the  Brahmaputra  River.  The  district  is  mountainous, 
and  mostly  covered  with  dense  forest;  the  towns  are 
small,  so  that  the  loss  of  life  was  less  serious  than  in 
many  less  powerful  shocks  in  more  populous  countries. 
Shillong,  the  chief  town  affected,  was  situated  almost  on 
the  eastern  edge  of  the  most  disturbed  area;  nevertheless 
it  was  completely  destroyed.  The  nature  of  the  shock 
there  caused  the  ground  to  jerk  backward  and  forward  for 
the  width  of  eighteen  inches  two  hundred  times  a  minute. 
Huge  trees  were  snapped  across  near  the  ground  as  the 
upper  parts  could  not  keep  pace  with  the  swift  backward 
and  forward  motion  of  the  lower  end  of  the  trunk.  Walls 
were  shaken  to  pieces,  and  the  roofs  settled  down  on  their 
debris.  Instructive  evidence  as  to  the  nature  of  the  earth- 
quake movement  was  afforded  by  the  overthrow  of  some 

106 


Earthquakes 


massive  standing  stones,  which  resemble  the  so-called 
Druidical  stones  of  western  Europe.  These  huge  stones 
are  very  numerous  in  the  Khasi  hills.  Some  of  the  stones 
with  a  pointed  base  resting  in  a  socket  were  flung  six  and 
a  half  feet  through  the  air ;  and  as  the  sides  of  the  socket 
were  undamaged,  the  stone  must  have  been  jerked  upward 
along  a  line  raised  at  least  60°  from  the  ground,  so  as  to 
be  thrown  clear  of  the  socket  (Fig.  10). 

Further  from  the  centre  of  the  earthquake  the  move- 
ments were  horizontal  rather  than  vertical,  as  was  shown, 
for  instance,  by  the  effect  on  the  railways ;  the  rails  were 
thrust  forward  until  they  were  buckled  into  sinuous  curves ; 


—  1 

f 

j 

/•""\  'ft 

&L_jC_-_ 

FIG.  io.— THROW  OF  THE  KHASI  MONUMENTS. 

many  of  the  bridges  collapsed  owing  to  the  girders  having 
been  pushed  forward  so  that  one  end  passed  off  the  pier 
and  fell.  Where  the  vertical  movement  of  the  ground 
was  more  pronounced,  some  of  the  wooden  bridges  were 
ruined  by  the  beams  having  been  driven  up  through  the 
roadway,  which,  on  their  descent,  they  dragged  down  with 
them. 

One  of  the  most  striking  results  of  the  Assam  earthquake 
was  the  clear  evidence  of  the  direct  disturbances  of  the 
surface  of  the  earth.  According  to  Dr.  Oldham,  the  earth- 
quake was  due  to  a  southward  thrust  of  the  area  along  a 
fault-plane  that  was  nearly  horizontal.  At  a  depth  of  five 
miles  below  the  surface,  the  rocks  being  soft  and  plastic, 

107 


Earthquakes 


there  was  probably  a  considerable  southward  movement  of 
material.  As  the  surface  consists  of  hard,  rigid  materials, 
no  great  movement  of  it  was  possible,  but  the  land  was 
compressed  from  north  to  south.  The  block  above  the 
moving  foundation  was  traversed  by  many  cracks  and 
fractures,  on  some  of  which  the  land  was  forced  upward, 
while  other  parts  were  thrown  into  gentle  folds. 

The  re-survey  of  the  area  showed  that  there  had  been 
an  uprise  of  the  surface  amounting  in  one  place  to  as 
much  as  twenty-four  feet.  The  bending  of  the  surface- 
was  proved  by  the  formation  of  new  lakes  in  the  hollows 
thus  caused.  Thus  the  valley  of  the  Rongtham  River  in 
the  Garo  Hills  was  buckled  so  that  one  reach  of  the  river 
was  lowered,  while  the  two  adjacent  parts  were  raised  ;  the 
water  collected  in  the  depression  and  formed  a  lake  half  a 
mile  long  and  twelve  feet  deep  at  a  point  where  the  depth 
at  an  old  ford  was  known  tc  have  been  only  one  foot. 
Also  along  the  course  of  the  Chedrang  River  the  land  was 
fractured,  and  the  land  on  the  eastern  side  of  this  fracture 
was  upraised  in  one  place  for  thirty-five  feet.  The  amount 
of  the  uplift  was  variable,  and  thus  produced  a  series  of 
dams  across  the  valley ;  lakes,  one  of  which  was  half 
a  mile  long  and  nearly  a  quarter  of  a  mile  wide,  were 
formed  in  the  depression  above  these  dams. 

The  changes  in  level  were  also  proved  by  the  views  in 
various  parts  of  the  country  having  been  altered,  owing  to 
the  lowering  of  the  intervening  hills.  Thus  from  a  point 
on  the  Garo  Hills,  whence  only  part  of  the  Brahma- 
putra River  had  previously  been  visible,  the  whole  width 
could  afterwards  be  seen. 

The  Assam  earthquake  therefore  had  a  direct  effect  on 
the  form  of  the  earth's  surface ;  the  folding  and  faulting 
produced  cliffs  and  hollows,  and  thrust  the  land  unequally 
southward. 

Earthquakes  are  often  famous  from  their  devastation 

108 


THE  MONUMENTS  IN  THE  WARNAMBOOL  CEMETERY  TWISTED 
BY  AN  EARTHQUAKE 


Earthquakes 


rather  than  from  their  power.  In  this  respect  the  most 
famous  of  all  earthquakes  was  that  of  Lisbon  on  ist  Novem- 
ber, 1755.  A  sound  like  thunder  was  heard,  and  then  a 
violent  shock  overthrew  parts  of  the  city.  The  streets 
were  narrow  and  the  buildings  tall,  and  to  escape  from 
falling  stones  the  people  fled  for  safety  on  to  the  open 
quay  beside  the  Tagus.  While  dense  crowds  were 
assembled  there,  an  earthquake  wave  first  sucked  the 
water  out  of  the  river  till  the  bar  across  the  mouth 
was  left  dry,  and  then  rushing  in  as  a  wave  fifty  feet 
above  the  ordinary  level,  it  swept  across  the  quay  and 
drowned  from  50,000  to  60,000  people.  The  quay  itself, 
a  solid  structure  built  of  marble,  sank  and  disappeared 
with  the  ships  beside  it.  It  is  said  that  no  fragments  of 
the  wrecks  were  ever  seen,  and  that  the  site  of  the  quay 
was  left  as  an  unfathomable  abyss.  According  to 
Humboldt,  the  area  shaken  by  the  earthquake  was  four 
times  as  large  as  Europe  ;  for  it  is  said  to  have  been  felt 
on  the  Swedish  coast,  to  have  temporarily  emptied  the  hot 
springs  of  Toplitz  in  Bohemia,  to  have  swallowed  up  a 
village  with  ten  thousand  inhabitants  eight  leagues  from 
Morocco,  and  to  have  raised  a  swell  on  the  Great  Lakes 
of  Canada,  of  Scandinavia,  and  on  Loch  Lomond. 

The  power  and  range  of  this  earthquake  have  probably 
been  greatly  exaggerated.  In  the  parts  of  Lisbon  built 
on  a  rocky  foundation,  not  a  building  was  injured.  The 
serious  damage  was  confined  to  the  areas  of  sand  and  clay, 
which  provided  a  weak  foundation.  The  destruction  of 
the  quay  was  probably  due  to  its  mass  being  too  heavy 
for  its  soft  foundations,  so  that  the  structure  was  easily 
destroyed  by  the  earthquake  wave,  and  the  stones  sank  into 
the  silt.  The  story  of  its  subsidence  into  a  deep  abyss 
may  be  rejected  as  inconsistent  with  the  records  of 
the  building  of  the  new  quay. 

The  range  over  which  the  Lisbon  earthquake  was  felt 

109 


Earthquakes 


has  also  been  overestimated.  The  hot  springs  at  Toplitz 
may  have  been  disturbed  by  the  earth  tremors,  as  hot 
springs  are  in  such  delicate  equilibrium  that  they  are 
easily  thrown  out  of  action ;  and  though  part  of  the  coasts 
of  the  North  Atlantic  may  have  been  reached  by  the 
earthquake  wave,  it  is  doubtful  whether  the  earthquake  was 
felt  on  land  farther  north  than  the  southernmost  part  of 
France. 

The  reported  range  of  the  earthquake  was  extended  by 
a  coincidence.  On  the  same  day  an  earthquake  was  felt 
in  Derbyshire,  and  the  waters  of  Loch  Lomond  in  Scot- 
land were  thrown  into  a  commotion  sufficient  to  produce 
a  series  of  waves.  The  belief  that  both  these  incidents 
were  caused  by  the  Lisbon  earthquake  was  natural ; 
but  as  it  was  not  felt  in  central  or  northern  France,  nor  in 
southern  England,  the  records  on  that  day  from  central 
England  and  Scotland  must  have  been  due  to  one,  or 
perhaps  two,  independent  earthquakes. 

The  San  Francisco  earthquake  of  1906  was  far  more 
disastrous  than  many  more  powerful  earthquakes ;  for  the 
line  of  earth  movement  which  caused  it  passed  through 
the  city  of  San  Francisco ;  but  most  of  the  devastation 
was  due  to  fires  which,  owing  to  the  breaking  of  the 
water-pipes,  could  not  be  controlled. 

This  earthquake  has  been  most  carefully  studied  by 
a  commission,  and  the  report,  edited  by  Prof.  A.  C. 
Lawson,  shows  that  the  earthquake  was  due  to  a  move- 
ment on  a  long  fault  which  runs  approximately  parallel  to 
the  Pacific  coast.  The  earthquake  was  felt  for  a  length  of 
over  700  miles,  and  for  a  distance  of  300  miles  to  the 
east  of  the  main  line,  so  that  it  was  probably  felt  over  an 
area  of  about  400,000  square  miles.  The  earthquake  was 
no  doubt  due  to  a  sudden  jerky  movement  of  the  land  along 
the  fault,  known  as  the  San  Andreas  Fault.  This  fault 
has  been  traced  for  a  length  of  over  600  miles,  and  the  San 

no 


Earthquakes 


Andreas  Lake  and  the  fiord  of  Tomales  Bay,  north-west 
of  San  Francisco,  both  occupy  depressions  along  it. 

The   earthquake   movements   ran   along  the  northern 
part  of  this  fault  for  a  length  of  about  270  miles,  between 


FIG.  ii. — LINE  OF  THE  FAULT  NEAR  SAN  FRANCISCO. 

a  locality  80  miles  south-east  of  San  Francisco  and 
Cape  Mendocino,  190  miles  to  the  north-north-west. 
The  land  on  the  western  side  of  the  fault  was  suddenly 
jerked  to  the  north,  while  the  land  on  the  eastern  side 

in 


Earthquakes 


was  moved  southward.  The  total  displacement  caused 
by  the  double  movement  varied  from  eight  to  twenty  feet, 
The  movement  was  most  clearly  shown,  as  in  some 
earlier  New  Zealand  earthquakes,  by  the  displacement  o 
fences.  Thus,  in  Fig.  13,  if  F  is  the  line  of  the  faull 
crossed  by  a  fence  which  before  the  earthquake  was 
situated  along  the  line  aa,  after  the  earthquake  it  was 
found  broken  across,  and  the  two  parts  left  on  the 


FIG.  12.  —  DISPLACEMENT  OF 
ROAD  BY  THE  FAULT  (F)  AT 
MINO-OWARI  EARTHQUAKE. 

a,  Original  position  of  road ; 
b,  position  of  road  after  earth- 
quake ;  the  road  at  bb  has  been 
lifted  20  feet,  as  well  as  moved 
to  the  north-west  (see  p.  114). 


FIG.  13.  —  DISPLACEMENT  OF 
FENCE  BY  EARTHQUAKE. 

jp,  Fault ;  a,  original  position  o 
fence  ;  b,  position  of  two  part 
of  fence  after  earthquake. 


lines  bb.  The  land  on  the  western,  or  Pacific,  side  of  the 
fault  also  appears  to  have  been  slightly  uplifted ;  anc 
the  uplift  produced  in  places  small  steplike  breaks  in  the 
land. 

The  San  Francisco  earthquake  was  the  last  of  a  seriei 
of  eight  which  happened  along  the  western  coast  o 
America  between  1899  and  1906  (Fig.  14).  The  serie: 
began  in  the  north,  off  the  coast  of  British  Columbia 
with  the  first  and  third  of  the  series,  in  September,  1899 

112 


Earthquakes 

and  October,  1900.  The  next  to  the  south— the  seventh, 
in  date — was  the  San  Francisco  earthquake  of  i8th  April, 
1906.  Still  farther  south  the  line  was  continued  off 


FIG.  14.— COURSE  OF  THE  EARTHQUAKE  LINE  ALONG 
WESTERN  AMERICA. 

Central  America  by  the  second,  fourth,  and  fifth  of  the 
series  in  January,  1900,  and  April  and  September,  1902. 
The  sixth  happened  off  northern  South  America,  upon 
the  southern  continuation  of  the  same  line,  on  3ist  January, 

113  H 


Earthquakes 


1906 ;  and  the  last  of  the  series,  the  southernmost,  on 
I7th  August,  1906,  devastated  Southern  Peru  and  Chile. 
The  course  of  this  doubtless  connected  series  of  great 
earthquakes  is  marked  on  Fig.  14. 

Earthquakes  not  only  cause  a  displacement  along  faults, 
but  buckle  and  compress  the  land  beside  them.  This 
effect  was  well  shown  by  the  earthquake  which,  on 
28th  October,  1891,  devastated  the  provinces  of  Owari  and 
Mino  in  Central  Japan.  These  provinces  consist  mainly 
of  fertile,  densely  populated,  alluvial  plains ;  the  earth- 
quake overthrew  the  houses,  opened  numerous  fissures  on 
the  ground,  destroyed  the  railway-lines,  and  started  great 
landslips  down  the  slopes  of  the  adjacent  mountains. 
The  two  most  remarkable  effects  were  that  the  river 
valleys  were  made  more  narrow,  as  if  the  two  banks  had 
been  pressed  together.  It  was  found  that  the  construction 
of  a  tube  railway  in  London  along  Cheapside  had  caused 
the  two  sides  of  the  street  to  approach  half  an  inch  nearer 
one  another.  During  the  Japanese  earthquake,  plots  of 
land  in  the  valley  of  the  Neo  were  compressed  from 
sixteen  to  ten  yards  in  width.  A  fault  seventy  miles  long 
was  formed  across  the  country  along  a  line  trending 
approximately  north-west  to  south-east ;  the  land  on  the 
fault  was  moved  both  vertically  and  horizontally.  The 
vertical  movement  was  most  marked  at  Midori,  in  the  Neo 
Valley,  twelve  miles  north-west  of  Gifu.  It  there  formed 
a  cliff  twenty  feet  high  by  upraising  the  north-eastern  side 
above  the  south-western  (Fig.  12)  ;  in  one  place  the  fault 
broke  across  a  road,  and  the  north-eastern  side  was  raised 
twenty  feet  high,  and  this  part  of  the  road  ended  abruptly 
against  the  foot  of  the  newly  formed  cliff.  The  movement 
along  the  fault  was  not  uniform,  for  along  most  of  the 
fault  the  movement  was  in  the  opposite  direction  to  that 
at  this  fractured  road,  and  thus  the  land  was  generally 
left  lower  on  the  north-eastern  side  of  the  fault. 

114 


Earthquakes 


FIG.  15. — DISTRIBUTION  OF  SOME  BRITISH  EARTHQUAKES  BETWEEN 
THE  YEARS  1884  AND  1910. 


Earthquakes 


The  British  area  is  often  regarded  as  unusually  free 
from  earthquakes,  which  are,  however,  more  numerous 
than  is  generally  thought.  The  map  (Fig.  15)  shows  the 
distribution  of  the  most  important  between  1884  and 
1910.  The  most  severe  British  earthquake  during  the 
last  four  centuries  occurred  in  East  Anglia,  with  its  centre 
near  Colchester,  on  22nd  April,  1884.  It  appears  to  have 
been  the  only  one  in  modern  times  which  occasioned  any 
direct  loss  of  life.  The  twisting  of  a  chimney  crushed 
a  bird's-nest  and  killed  a  starling !  The  earthquake  did 
considerable  damage  in  the  Colchester  district. 

The  intensity  of  the  different  earthquakes  is  shown  on 
the  map  by  their  numbers,  on  what  is  known  as  the 
Rossi-Ferol  scale.  This  classifies  earthquakes  into  ten 
grades,  as  follows : 

Nos.  i  and  2 — known  only  from  records  on  special 

instruments. 

No.  3 — felt  by  persons  at  rest. 
No.  4 — felt  by  persons  in  motion  ;  ceilings  cracked. 
No.  5 — felt  by  everybody;  delicately  hung  bells  rung. 
No.  6 — awakens  sleepers;  trees  visibly  shaken. 
No.  7 — movable  objects  overthrown  ;  falls  of  plaster. 
No.  8 — chimneys  overthrown. 
No.  9 — buildings  destroyed. 
No.  10 — the  ground  fissured;  falls  of  rocks;  general 

widespread  devastation. 

Judged  by  this  scale,  the  Glasgow  earthquake  of  1910 
was  of  the  grade  No.  5^ ;  the  Inverness  earthquake  of 
1901,  No.  8 ;  and  the  Colchester  earthquake,  No.  8J. 

The  map  (Fig.  15)  indicates  the  dependence  of  British 
earthquakes  on  the  great  faults  which  traverse  the  country. 
The  Inverness  earthquake  of  1901  and  that  of  Strontian 
in  Argyle  in  1902  happened  along  two  important  Scottish 
faults,  which  extend  from  north-east  to  south-west.  The 
line  most  affected  by  the  Loch  Broom  earthquake  of 

1x6 


Earthquakes 


1889  trends  almost  at  right  angles  to  those  of  the  two 
others,  because  it  extended  along  the  Loch  Broom  faults, 
which  trend  from  north-west  to  south-east.  The  most 
frequent  British  earthquakes  happen  near  Crieff  and 
Comrie,  two  localities  beside  the  Highland  Boundary 
Fault  which  separates  the  Highlands  from  the  Midland 
Valley  of  Scotland,  the  slight  earthquakes  that  are 
constantly  reported  along  that  line  showing  that  some 
movement  is  still  taking  place  along  that  ancient  fault. 

The  Colchester  earthquake  appears  to  have  been  an  ex- 
ception to  the  rule  which  connects  earthquakes  with  well- 
marked  faults.  It  was  probably  due  to  a  concealed  fault, 
for  evidence  from  wells  and  the  high  level  of  some  Essex 
gravels  indicate  that  faulting  and  earth  movements  have 
taken  place  along  the  line  most  disturbed  by  the  earthquake. 

On  i4th  December,  1910,  the  Glasgow  district  was  sharply 
shaken  by  a  shock,  which  was  felt  by  people  over  an 
area  of  about  320  square  miles.  It  was  probably  due  to 
fresh  sinking  on  a  pair  of  faults  which  pass  through  the 
northern  suburbs  of  Glasgow.  British  earthquakes, 
though  not  infrequent,  are  weak  and  gentle. 


Lschi 


FIG.  16. — THE  BAY  OF  NAPLES.    (See  p.  102.) 
117 


CHAPTER  VIII 
VOLCANOES 

THE  inhabitants  of  the  Mediterranean  islands  near 
southern  Italy  and  Greece  are  familiar  with  mountains 
which,  instead  of  maintaining  a  constant  majestic  repose, 
give  forth  vast  columns  of  steam,  flare  at  night  like 
fires,  are  shaken  by  deep  explosions,  hurl  forth  volleys 
of  hot  stones,  and  discharge  streams  of  molten  rock.  The 
separation  of  these  violent  eruptions  by  intervals  of  rest, 
and  the  resemblance  of  the  accompanying  sounds  to  the 
blasts  of  a  mighty  bellows,  all  suggested  comparison  with 
a  smithy  fire ;  and  the  mountains  were  poetically  explained 
as  the  chimneys  above  the  fires  of  Vulcan,  the  smith  of  the 
infernal  regions.  The  best  known  of  these  violent  moun- 
tains, one  of  the  Lipari  Islands  north  of  Sicily,  was  there- 
fore named  "Vulcano,"  and  all  such  mountains,  whether 
still  active  or  extinct,  are  now  known  as  "  volcanoes." 
Mountains  composed  of  somewhat  similar  materials,  but 
not  now  in  active  operation,  were  found  in  other  parts  of 
Europe  ;  and  as  their  rocks  had  clearly  been  once  molten 
like  slag,  medieval  authorities  explained  them  as  the  slag- 
heaps  of  Roman  iron  smelters.  Still  later  some  of  the 
founders  of  modern  geology  explained  volcanoes  as  due  to 
the  action  of  fire,  for  some  of  them,  confident  that  all 
rocks  were  due  to  the  action  of  water,  rejected  the  theory 
that  any  were  due  to  an  internal  source  of  heat.  Cliffs  of 
shale,  which  contain  iron  pyrites  and  are  mixed  with  either 
fragments  of  coal  or  some  other  inflammable  material,  are 

1x8 


Volcanoes 

often  set  on  fire  by  the  decomposition  of  the  pyrites ;  and 
these  burning  cliffs  sometimes  smoulder  for  years.  Hence 
it  was  thought  that  the  more  violent  fires  of  volcanoes 
were  due  to  the  combustion  of  seams  of  coal,  melting 
layers  of  overlying  and  easily  fused  rock.  Hence  geology 
adopted  such  terms  as  "smoke,"  "flame,"  "ashes,"  and 
"  cinder  cones,"  which  were  used  in  the  description  of  vol- 
canoes when  they  were  accepted  as  burning  mountains.  But 
this  idea  of  the  nature  of  a  volcano  is  entirely  misleading. 
Burning  is  a  process  whereby  a  material,  which  acts 
as  a  fuel,  combines  with  some  other  constituent,  usually 
the  oxygen  of  the  atmosphere.  Burning  is  a  chemical 
process ;  but  a  volcanic  eruption  is  not  due  to  combustion. 
The  material  may  be  made  so  hot  as  to  melt ;  but  it  is  a 
change  of  condition  and  not  of  composition.  Volcanic 
rocks  are  melted,  not  burnt. 

It  is  said  that  the  earliest  explorer  who  saw  Mt.  St. 
Elias  estimated  its  height  as  17,000  feet,  and  that  his 
immediate  successor  reduced  this  to  16,000;  the  next 
to  15,000 ;  and  each  following  visitor  took  off  1,000  feet, 
until  one  at  length  declared  that  Mt.  St.  Elias  was 
really  a  hole  in  the  ground.  That  was  his  playful  way  of 
expressing  his  belief  that  the  mountain  was  a  volcano, 
a  view,  however,  which  was  a  mistake.  But  this  statement 
included  a  correct  interpretation  of  the  nature  of  a  volcano ; 
for  it  is  a  hole  or  pipe  through  which  material  from  below  the 
surface  of  the  earth  can  be  forced  to  the  surface,  and  there 
escape  either  in  explosions  or  in  floods  of  molten  rock. 

The  rocks  are  melted  by  the  earth's  internal  heat. »  The 
pressure  of  the  overlying  rocks  prevents  the  deep-seated 
material  becoming  molten,  though  it  is  fluid ;  but  if 
any  fissure  be  opened  by  which  the  overheated  fluid  rock 
can  flow  to  the  surface,  it  will  rise  until  the  pressure  is  so 
reduced  that  the  rock  can  become  molten  and  be  discharged 
in  the  form  of  lava. 

IIQ 


Volcanoes 

The  presence  of  lava  is,  however,  not  indispensable  in 
volcanic  eruptions.  Thus  no  stream  of  lava  was  dis- 
charged either  in  the  first  recorded  eruption  of  Vesuvius 
(A.D.  79),  or  in  that  of  Tarawera  in  New  Zealand  in  1886. 
The  molten  rock  is  usually  so  saturated  with  gas  that 
when  the  pressure  is  released  at  the  surface,  the  gases 
explode  and  the  lava  is  blown  into  fragments.  The  largest 
of  the  blocks  of  lava  thus  scattered  fall  immediately  around 
the  vent  of  the  volcano  and  form  the  rock  known  as 
"  agglomerate."  The  smaller  fragments,  which  range  in 
size  from  that  of  a  cocoanut  down  to  that  of  a  peanut,  are 
known  as  "volcanic  scoria,"  or  more  familiarly  as  "vol- 
canic ash."  Scoria  is  blown  farther  from  the  vent  than 
the  agglomerate,  and  it  accumulates  as  a  volcanic  moun- 
tain around  the  vent,  or  still  farther  away  it  forms  thin, 
widespread  sheets  of  fine-grained  scoria.  The  finest 
material  ejected  is  carried  far  afield  by  the  wind,  and  falls 
as  a  fine  powder  known  as  "  volcanic  dust."  Some  of  this 
dust  is  so  fine  that  it  floats  in  the  air  for  months  after 
the  eruption,  and  may  be  carried  by  wind  in  the  upper 
atmosphere  to  all  parts  of  the  world.  Thus  the  exquisite 
sunsets  which  were  visible  in  Europe  in  the  autumn  of 
1883  were  due  to  volcanic  dust  that  had  been  blown  into 
the  atmosphere  at  the  explosion  of  Mt.  Krakatoa  in  the 
Straits  of  Sunda  between  Java  and  Sumatra. 

Immediately  around  a  volcano  there  are  also  laid  down 
beds  of  volcanic  mud.  The  moisture  discharged  in  the 
eruption  may  be  carried  several  miles  above  the  earth's 
surface.  It  is  there  condensed  into  rain,  which  falls 
through  the  dust-laden  atmosphere,  washes  the  dust  out 
of  it,  and  deposits  it  in  streams  and  sheets  of  mud.  In 
the  eruption  of  Vesuvius  in  A.D.  79,  Pompeii  was  covered 
by  volcanic  ash,  while  Herculaneum  was  more  securely 
buried  under  a  sheet  of  volcanic  mud. 

The  nature  of  volcanic  processes  is  well  illustrated  by 

120 


THE  VOLCANIC  CONE  OF  SMERO,  JAVA 

Showing  the  cloud  of  heavy  dust-laden  steam  ;  the  rounded  bosses  on  the  sides  of  the 
cloud  are  compared  to  the  form  of  a  cauliflower. 


Volcanoes 

reference  to  that  water-volcano,  a  geyser.  The  word 
"geyser"  is  Icelandic,  and  means  "to  gush,"  "to  rage,"  or 
"  to  break  suddenly,"  and  it  is  now  used  for  explosive  hot 
springs.  The  Icelandic  geysers  are  situated  about  seventy 
miles  south-eastward  from  Reykjavik,  the  chief  town 
on  the  island,  and  about  thirty  miles  from  Hekla,  the 
most  famous  of  Icelandic  volcanoes.  The  geysers  belong 
to  a  group  of  some  seventy  hot  springs  situated  in  a  band 
about  a  third  of  a  mile  long.  Three  of  these  hot  springs — 
the  Great  Geyser,  Little  Geyser,  and  Strokr — sometimes 
discharge  their  water  by  sudden  explosions. 

Each  geyser  consists  of  a  well,  which  may  be  a  cylin- 
drical tube  or  a  series  of  irregular  cavelike  chambers.  It 
discharges  above  through  a  vent,  which  is  surrounded 
by  a  mound  of  siliceous  sinter.1  This  material  has  been 
derived  from  the  hot  geyser  water.  In  the  centre  of  the 
mound,  above  the  vent,  is  a  saucer-shaped  hollow,  known 
as  the  "  basin,"  which  corresponds  to  a  volcanic  crater. 
The  Great  Geyser  has  a  basin  four  feet  deep  and  sixty  feet 
wide  ;  its  mound  is  twenty  feet  high,  and  the  well  is  ten 
feet  in  diameter,  and  between  seventy  to  eighty  feet  deep. 
The  basin  is  usually  filled  with,  hot  water,  the  overflow 
from  which  is  discharged  at  intervals  over  the  rim,  when 
the  level  is  raised  by  boiling.  At  irregular  intervals,  which 
are  now  much  longer  than  formerly,  the  water  boils  with 
great  violence,  and  the  geyser  is  emptied  by  an  explosion, 
which  throws  the  water  to  a  height  of  from  one  hundred 
to  over  three  hundred  feet  above  the  vent. 

The  explanation  of  these  geyser  eruptions  is  founded  on 
the  discovery  by  the  French  geologist,  Robert,  that  the 
water  at  the  bottom  of  the  geyser  well  is  hotter  than  the 
water  boiling  in  the  basin.  Water  boils  at  sea-level  at 
the  temperature  of  212°  F.  It  boils  on  the  summit  of  Mont 
Blanc,  15,780  feet  high,  at  the  temperature  of  183°  F.;  and 
it  would  not  boil  until  the  temperature  reached  220°  F.  at 

121 


Volcanoes 

the  bottom  of  a  mine,  4,000  feet  below  sea-level.  Water 
boils  at  a  cooler  temperature  on  a  mountain  summit,  and 
at  a  higher  temperature  in  the  bottom  of  a  mine,  owing  to 
the  difference  in  the  pressure  of  the  air.  Water  at  the 
temperature  of  212°  in  a  deep  mine  remains  liquid  instead 
of  passing  away  as  steam,  because  of  the  pressure  of  the 
extra  height  of  air  above  it.  For  the  same  reason  water 


25* 


...R5.  . 

....... 

....$?..... 

...5.9... 


FIG.  17.— SECTION  ACROSS  THE  GREAT  GEYSER  OF  ICELAND. 

Central  column  of  figures  shows  depths  in  feet ;  column  of  figures  to  left 
shows  temperature  shortly  before  eruption  ;  column  of  figures  to  right 
shows  temperature  at  those  depths  at  which  the  water  burst  into  steam. 

can  be  heated  in  a  closed  vessel  to  temperatures  much 
higher  than  its  boiling-point  in  an  open  vessel.  Water  at 
a  temperature  higher  than  that  at  which  it  would  boil 
under  ordinary  pressure  is  said  to  be  "  superheated." 
The  water  at  the  bottom  of  a  geyser  well  is  superheated. 

At  the  depth  of  seventy-one  feet  in  the  Great  Geyser 
(Fig.  17)  the  water  rises  before  boiling  to  the  temperature 
of  275°,  owing  to  the  pressure  of  the  overlying  column  of 

122 


Volcanoes 

water ;  but  if  it  be  raised  to  a  still  higher  temperature,  or 
if  the  pressure  of  the  water  above  it  be  reduced,  then  it 
will  instantly  expand  into  steam.  According  to  one  theory, 
that  of  Descloiseaux,  the  geyser  water  is  poured  into  the 
bottom  of  the  geyser  at  temperatures  of  over  276° ;  but  if 
the  well  be  empty,  the  water  is  immediately  cooled  below 
that  temperature  by  the  chilling  effect  of  the  walls.  But 
as  the  tube  is  gradually  filled  by  the  discharge  into  it  of 
water  at  the  temperature  of  over  276°,  the  temperature 
at  the  bottom  gradually  rises  above  275°,  and  then  the 
lowest  water  bursts  into  steam,  and  uplifts  all  the  water  in 
the  column  above  it.  The  water  in  the  top  of  the  geyser 
tube  overflows,  and  the  water  in  the  rest  of  the  tube  is 
raised  to  levels  at  which  the  pressure  is  insufficient  to 
keep  it  liquid.  So  the  whole  column  of  water  instantly 
explodes  into  steam.  Hence  the  explosion  at  the  bottom 
of  the  well  causes  the  simultaneous  explosion  of  all  the 
water  in  it.  The  water  in  the  basin  is  hurled  into  the  air. 
Some  of  it  falls  back  into  the  basin  and  runs  into  the  well, 
whence  it  is  expelled  by  successive  explosions,  until,  by 
the  cooling  of  the  tube  and  the  loss  of  water,  the  eruption 
gradually  ceases. 

According  to  a  modification  of  this  theory  by  Bunsen, 
the  explosions  are  not  due  to  the  superheating  of  the 
bottom  layer,  but  to  the  circulation  of  water  in  the  tube. 
If  a  lump  of  turf  be  thrown  into  a  geyser,  it  carries  some  of 
the  water  down  with  it,  and  accordingly  forces  some  of 
the  water  up  the  well.  The  rising  water  passes  to  a  level, 
at  which  the  pressure  is  no  longer  sufficient  to  prevent  its 
explosion  ;  and  explosion  at  the  one  point,  by  uplifting 
the  water  above  it  and  reducing  the  pressure  on  the  water 
below,  enables  the  whole  of  the  superheated  water  to  pass 
at  once  into  steam. 

The  processes  which  control  geyser  eruptions  are  well 
illustrated  by  some  of  the  smaller  geysers  in  New  Zealand. 

123 


Volcanoes 

Thus  the  geyser  known  as  the  Feathers,  in  the  geyser 
basin  of  Wairakei,  can  be  kept  quiet  by  pouring  cold  water 
into  it.  When  the  cold  water  is  turned  off,  the  geyser 
water  rises  in  temperature  until  it  is  superheated,  and  an 
eruption  occurs.  An  adjacent  eruptive  spring,  known  as 
the  Lightning  Pool,  is  controlled  by  varying  the  height  of 
water.  It  has  a  basin  about  five  feet  deep,  in  which  the 
temperature  of  the  water  is  usually  204° — the  level  of 
Wairakei  being  2,200  feet  high  above  sea-level,  the  normal 
boiling-point  there  is  about  208°.  By  opening  a  gap  in  the 
rim  of  the  Lightning  Pool,  the  level  of  the  water  is  lowered 
four  inches,  and  this  reduction  in  pressure  produces  a 
miniature  eruption. 

There  are  three  chief  geyser  fields — Iceland,  the  Yellowr- 
stone  Park  in  the  Rocky  Mountains,  and  the  North  Island 
of  New  Zealand.  But  geysers  depend  on  a  very  delicate 
adjustment  of  temperature  and  pressure ;  it  is  not  then 
surprising  that  their  life  is  short.  There  is  no  reference 
to  them  in  the  ancient  Icelandic  sagas ;  this  makes  it 
credible  that  those  in  Iceland  have  developed  in  modern 
times,  and  as  their  eruptions  are  now  more  irregular  and 
comparatively  infrequent,  they  may  be  already  passing 
into  the  condition  of  non-explosive  hot  springs. 

The  great  geysers  of  the  Yellowstone  Park  are  also  less 
regular  in  action  than  formerly.  The  most  powerful  of 
all  known  geysers  was  Waimangu,  the  Black  Geyser,  in 
New  Zealand,  which  was  situated  at  the  bottom  of  the 
long  chasm  formed  by  the  explosion  of  Tarawera  in  1886 
(see  pp.  134, 135).  It  was  first  seen  in  eruption  in  February, 
IQOI.  Its  well  was  eighty  feet  deep,  and  it  shot  up  a 
column  of  water,  which,  owing  to  the  quantity  of  mud 
and  stones  ejected  with  it,  was  of  inky  blackness.  After 
a  period  of  rest  the  water  has  been  thrown  to  a  height 
of  i, 600  feet  above  the  mouth  of  the  geyser.  In  1905  the 
adjacent  lake,  Rotomahana,  the  level  of  which  had  been 

124 


Volcanoes 

slowly  rising,  burst  its  barrier  and  was  nearly  emptied. 
The  water-level  in  the  adjacent  ground  was  thereby  lowered, 
and  Waimangu  became  extinct. 

In  volcanoes,  as  in  geysers,  the  essential  part  is  the 
pipe  by  which  the  overheated  material  is  raised  to  the 
earth's  surface.  The  upper  end  of  this  pipe  is  known 
as  the  "vent,"  or  "mouth,"  and  the  material  discharged 
through  it  collects  around  it  in  a  ring-shaped  hill.  Within 
this  ring  there  is  a  cup-shaped  hollow,  known  as  the 
"  crater,"  which  in  ordinary  volcanoes  has  steep  walls. 

In  some  volcanoes,  however,  the  vent  opens  above  into 
a  much  larger  and  proportionally  shallower  depression, 
which  has  therefore  been  appropriately  called  a  "  caldron." 
The  term  was  introduced  to  geology  in  the  Spanish  form 
of  caldera  (a  pot — calderon,  a  big  pot)  for  the  volcanic 
basins  in  the  Hawaiian  Islands. 

True  craters  are  formed  either  by  the  building  up  of 
a  ring-shaped  mountain  of  lava,  or  scoria,  or  both,  around 
the  volcanic  vent ;  or  by  the  excavation  of  a  deep  pit 
above  the  vent  by  a  volcanic  explosion.  Caldrons  are 
due  to  the  subsidence  of  the  ground  around  the  vent. 
They  are  often  occupied  by  lakes.  The  sinking  may  be 
caused  by  the  collapse  of  the  subterranean  cavities  left 
after  the  discharge  of  lava,  or  by  the  shrinkage  while 
cooling  of  the  material  in  and  around  the  pipe. 

Caldrons  are  distinguished  from  craters  by  the  fact 
that  they  are  often  much  larger,  and  are  shallower  in 
proportion  to  their  size.  Caldrons  are  usually  found  in 
volcanoes  built  up  of  lavas,  such  as  basalt,  which  are  dis- 
charged quietly  and  not  explosively,  whereas  the  great 
explosion  craters  are  mostly  found  in  volcanoes  composed 
of  such  lavas  as  andesites.  It  is  probable  that  many  of 
the  circular  basins  on  the  moon,  which  are  usually  called 
"  craters,"  have  been  formed  as  caldrons. 

125 


Volcanoes 

The  distribution  of  volcanoes  on  the  earth's  surface 
throws  much  light  on  their  origin.  The  first  striking  fact 
apparent  from  a  map  of  the  distribution  of  volcanoes  (see 
Fig.  9,  p.  103)  is  that  they  are  mostly  arranged  in  lines. 
Thus  in  South  America  volcanoes  occur  at  intervals  along 
the  course  of  the  Andes,  while  they  are  absent  from  the 
rest  of  South  America.  There  are  three  approximately 
equidistant  volcanic  lines  running  north  and  south  across 
the  earth.  The  first  runs  down  the  western  side  of 
America  from  Alaska  to  Patagonia.  The  next  runs  from 
Iceland,  through  the  ancient  volcanic  centres  of  Scotland, 
up  the  Rhine  Valley,  across  the  volcanic  fields  of  Italy, 
and  down  the  Red  Sea  to  eastern  Africa  and  Madagascar. 
The  third  great  line  begins  in  Burmah  and  the  Bay  of 
Bengal,  and,  like  the  others,  trends  eastward  as  it  goes 
south ;  it  passes  through  the  Malay  Archipelago,  the 
islands  of  Melanesia  and  New  Zealand,  and  continues  to 
South  Victoria  Land.  This  line  is  joined  by  a  branch 
from  Japan  and  the  Philippines. 

In  addition  to  these  three  main  lines  there  are  some 
isolated  volcanic  lines  and  single  volcanoes.  The  chain 
of  volcanoes  on  the  eastern  side  of  the  West  Indies  has 
been  built  up  along  the  line  separating  the  Caribbean 
subsidence  from  the  Atlantic.  The  volcanic  islands  of 
the  Atlantic,  Southern,  and  Pacific  Oceans  are  probably 
on  lines  of  weakness  which  only  gave  rise  to  volcanoes  at 
one  or  a  few  isolated  points. 

The  volcanoes  are  not  always  arranged  regularly  along 
the  main  lines ;  they  often  occur  in  groups  separated  by 
wide  areas  of  non-volcanic  rocks.  The  groups,  moreover, 
may  not  be  linear ;  for  they  are  arranged,  as  in  the  case 
of  the  volcanoes  of  southern  Italy,  along  lines  radiating 
from  the  centre  of  a  sunken  area ;  and  in  many  cases  the 
volcanoes  occur  along  short  lines  transverse  to  the  main 
line.  For  example,  in  Central  America  the  chief  volcanic 

126 


TRAIN  RAILS  BUCKLED  BY  AN  EARTHQUAKE 

The  rails  on  a  railway  near  San  Bruno,  California,  buckled  by  compression  during 
the  San  Francisco  earthquake,  1906. 


A  SMALL  CRATER 

Earthquake   crater  near   Watsonville,    made   by   the   San    Francisco    earthquake. 
The  foot-rule  on  the  further  side  gives  the  scale. 


Volcanoes 

lines  are  arranged  east  and  west,  and  are  not  parallel  to 
the  main  axis  of  the  country.  Java  extends  east  and 
west ;  but  its  volcanoes  often  occur  on  lines  running 
from  north-west  to  south-east,  and  the  chief  volcanic 
series  in  Japan  are  also  across  the  main  axis  of  the 
archipelago. 

After  the  linear  arrangement  of  volcanoes  the  second 
striking  point  in  their  distribution  is  their  proximity  to 
the  sea.  As  a  rule  they  are  on  the  coast-lands  or  on 
islands,  and  this  fact  naturally  led  to  the  conclusion  that 
the  volcanoes  were  necessarily  connected  with  the  sea. 
Steam  has  been  generally  regarded  as  the  main  explosive 
agent  in  volcanic  eruptions ;  and  the  steam  was  attributed 
to  the  evaporation  of  sea-water,  which  was  thought  to  find 
its  way  through  fissures  into  contact  with  intensely  heated 
rocks.  The  sudden  conversion  of  water  into  steam  was 
regarded  as  the  cause  of  the  eruptions.  This  hypothesis 
?  was  at  one  time  supported  by  the  claim  that  marine  fossils 
are  found  in  lavas ;  but  this  conclusion  was  based  on  minute 
fossils  which  had  been  accidentally  mixed  with  the  lavas, 
and  also  on  blocks  of  fossil-bearing  rocks  brought  up  by 
the  lavas  from  beneath  the  volcano. 

The  one  valid  argument  in  favour  of  the  connection  of 
volcanoes  with  the  sea  is  that  based  on  geographical  dis- 
tribution ;  but  the  prevalent  trend  of  opinion  has  been  to 
the  view  that  this  connection  is  a  coincidence.  There  are 
some  important  exceptions  to  the  rule.  Some  volcanoes 
occur  at  considerable  distances  from  the  sea.  Thus 
Cotopaxi  is  150  miles  inland;  and  it  seems  improbable 
that  if  water  from  the  Pacific  reached  hot  subterranean 
layers,  the  eruptions  would  be  produced  so  far  inland  and 
on  the  lofty  summits  of  the  Andes.  They  would  rather 
be  expected  on  the  lowlands  near  the  coast.  Further,  the 
Teleki  Volcano  in  British  East  Africa  is  450  miles  from 
the  sea.  True  it  is  near  Lake  Rudolf,  but  this  volcano 

127 


Volcanoes 

and  others  which  are  still  steaming  are  situated  along  the 
Great  Rift  Valley,  and  are  probably  due  to  the  earth 
movements  which  made  that  valley.  The  basin  of  Lake 
Rudolf  is  probably  due  to  the  same  series  of  movements 
as  the  volcano  instead  of  being  its  cause. 

The  reason  why  volcanoes  are  near  the  sea  is  because 
they  occur  on  the  borders  of  sunken  areas.  If  pressure  be 
applied  to  a  closed  tin  full  of  water,  the  pressure  may  cause 
some  of  the  water  to  ooze  out  along  the  seams  of  the  tin. 
If  a  great  block  of  the  earth's  crust  subsides,  the  pressure 
on  the  underlying  plastic  layer  of  the  crust  will  cause 
some  of  this  material  to  flow  outward,  and  this  may 
escape  in  volcanic  eruptions  at  weak  points  in  the  fissures 
bounding  the  sinking  area. 

The  pressure  of  the  slowly  sinking  floor  may  force  the 
lava  to  rise  slowly  and  steadily  up  a  volcanic  pipe,  and 
discharge  periodically  in  quiet  overflows,  as  in  the  vol- 
canoes of  the  Sandwich  Islands.  "  Can  we  conceive," 
says  Lowthian  Green,  "  of  any  force  more  in  accordance 
with  this  slow,  secular  rise  of  molten  matter  in  the 
Hawaiian  volcanoes  than  the  slow  subsidence  of  the 
earth's  crust  into  it — the  subsidence  of  the  bed  of  the 
Pacific  ?"  2 

*  Less  regular  subsidences  occasion  eruptions  at  irregular 
intervals,  and  the  sudden  yielding  of  the  weak  places  in 
the  crust  may  lead  to  short  explosive  eruptions. 

The  relations  between  earth  movements  and  volcanic 
eruptions  have  been  often  well  illustrated  by  the  volcanic 
history  of  the  West  Indies  (Fig.  18).  The  great  disasters 
of  1902  apparently  began  by  the  sinking  of  some  part  of 
the  West  Indian  area.  For  the  previous  twelve  months 
St.  Vincent  had  been  shaken  by  earthquakes.  On 
i8th  April,  1902,  there  was  apparently  a  sudden  collapse, 
for  Quezaltenango,  the  commercial  capital  of  Guatemala, 
was  then  overthrown  by  an  earthquake.  Five  days  later 

128 


Volcanoes 

lanche  on  the  doomed  city.  The  inhabitants,  numbering 
about  35,000,  practically  all  perished.  Every  building  in 
the  city  was  overthrown,  though  in  the  southern  part 
some  walls  parallel  to  the  path  of  the  volcanic  blast  were 
left  standing.  The  ships  in  the  harbour  were  all  sunk 
with  the  exception  of  one,  the  Roddam ;  the  captain 
slipped  anchor,  and  skilfully  navigated  backward  out  to 
sea.  As  the  volcanic  dust  on  board  the  Roddam  set  fire 
to  cotton  goods  and  oily  waste,  but  not  hemp  or  wool,  the 
temperature  of  the  cloud  when  it  struck  the  steamer  is 
approximately  known. 

Both  Mont  Pele"e  and  the  Soufriere  of  St.  Vincent  are 
composed  of  andesite,  the  rock  so  often  associated  with 
violent  explosive  eruptions,  and  they  neither  of  them 
discharged  any  stream  of  lava,  only  showers  of  scoria. 
Mont  Pelee,  however,  while  cooling  down  from  the  erup- 
tion, displayed  a  unique  phenomenon,  which  indicates 
that,  after  the  explosive  stage,  molten  material  was  being 
forced  up  the  volcanic  pipe  by  slow  heavy  pressure. 

A  colossal  column  of  lava  was  pushed  up  out  of  the 
pipe,  and  for  some  months  stood  like  a  monument  above 
the  fatal  mountain.  It  broke  to  pieces  before  the  ground 
around  it  had  cooled  enough  for  the  pillar  to  be  reached. 

It  was  doubtless  a  plug  of  lava  that  had  solidified  in 
the  pipe  of  the  volcano,  and  had  been  forced  upward,  just 
as  a  cork  may  be  partially  pushed  from  the  neck  of  a 
bottle.  The  steady  pressure,  like  that  which  causes  the 
slow  ascent  of  the  molten  lava  at  Kilauea,  was  probably 
due  to  the  sinking  of  some  neighbouring  part  of  the 
earth's  crust. 

A  somewhat  similar  sequence  of  events  happened  in 
1812  on  the  previous  great  eruption  of  the  Soufriere  of 
St.  Vincent.  At  the  end  of  1811  the  West  Indies  and 
Central  America  were  shaken  by  earthquakes,  and  one 
of  them,  on  the  26th  March,  1812,  devastated  Caracas,  the 


Volcanoes 

capital  of  Venezuela ;  and  on  the  24th  April  the  Soufriere 
of  St.  Vincent  suddenly  burst  into  eruption.  The  accounts 
show  that  the  violent  eruption  was  an  explosion  of  the 
same  general  type  as  that  of  1902.  Hence,  in  both  1812 
and  1902  violent  explosive  eruptions  of  the  volcanoes  on 
the  eastern  side  of  the  West  Indies  were  preceded  by 
widespread  earthquakes,  culminating  in  an  especially  dis- 
astrous shock  on  a  more  western  part  of  the  area. 

Volcanoes  are  situated  beside  areas  of  recent  earth 
movement  and  of  widespread  subsidence ;  and  as  the 
greater  subsidences  have  been  occupied  by  the  oceans, 
the  coincidence  of  the  volcanic  lines  with  the  ocean 
borders  can  be  explained  independently  of  the  action  of 
sea-water. 

The  last  volcanoes  that  were  in  action  in  the  British 
Isles  built  the  volcanic  plateau  of  north-eastern  Ireland, 
and  piled  up  a  series  of  great  volcanic  cones  in  the  islands 
of  Mull,  Skye,  and  some  other  western  islands,  and  on  the 
Ardnamurchan  Peninsula.  These  volcanic  eruptions  were 
no  doubt  connected  in  origin  with  those  of  the  Faroe 
Islands  and  of  the  great  volcanic  plateau  of  Iceland. 
This  chain  of  volcanic  groups  was  probably  due  to  the 
eruption  of  material  displaced  by  the  subsidence  of  the 
North  Atlantic  basin. 

One  of  the  main  problems  in  connection  with  volcanic 
action  of  especial  interest  in  the  present  time  concerns 
the  place  of  steam  as  the  explosive  agent.  The  immense 
quantities  of  steam  discharged  in  volcanic  eruptions  early 
attracted  attention.  Fouque  calculated,  for  example,  that 
in  the  eruption  of  Etna  in  1865  nearly  three  million  cubic 
yards  of  water  were  discharged  as  steam  from  a  secondary 
crater  on  the  side  of  the  mountain,  in  addition  to  the 
vast  quantities  that  escaped  from  the  main  crater.  The 
deluges  of  rain  which  often  fall  during  eruptions  are 
largely  formed  from  the  steam  discharged  from  the 


Volcanoes 

volcano.  Hence,  many  geologists  maintain  that  expand- 
ing steam  is  the  main  agent  in  the  ascent  of  lava  up  the 
pipe  of  a  volcano,  and  that  its  sudden  release  is  the  most 
powerful  factor  in  volcanic  explosions.  It  is,  however, 
claimed  by  another  school  of  vulcanologists  that  the 
escaping  steam  is  simply  due  to  surface  water,  and  that  it 
is  an  effect  and  not  the  cause  of  the  volcanic  eruption. 
Dr.  Brun  of  Geneva  has  especially  advocated  the  view 
that  volcanic  eruptions  are  independent  of  the  action  of 
water.  He  has  analyzed  the  gases  discharged  from 
volcanoes  in  many  parts  of  the  world,  and  claims  that 
water  is  not  an  essential  constituent ;  and  he  has  shown 
that,  under  certain  conditions,  the  clouds  that  arise  from 
Kilauea  in  the  Hawaiian  Islands  are  composed  of  solid 
material,  and  not  of  steam.  Dr.  Brun  goes  so  far  as  to 
maintain  that  the  deep-seated  igneous  rocks  contain  no 
original  water,  and  that  the  species  of  minerals  containing 
water  which  are  found  in  granite  are  of  secondary  origin  ; 
hence,  according  to  him,  the  micas  were  not  original 
constituents  of  granite,  but  have  been  formed  subse- 
quently, when  water  from  the  surface  of  the  earth 
penetrated  into  the  granite.  According  to  Brun,  the 
gases  which  cause  volcanic  eruptions  are  the  following : 
carbon  dioxide,  carbon  monoxide,  hydrochloric  acid, 
chlorine,  sulphurous  acid,  with  some  hydrogen,  oxygen, 
and  nitrogen. 

These  views  appear,  however,  improbable.  That  many 
volcanoes  discharge  less  steam  than  others  has  long  been 
recognized  by  geologists,  and  that  the  volcanic  cloud 
above  Kilauea  may  sometimes  consist  of  dry,  solid  dust  is 
not  surprising,  since  Kilauea  was  described  by  Lowthian 
Green,  for  example,  as  a  volcano  whose  lavas  do  not  emit 
steam.  He  observed  repeated  eruptions  of  Kilauea  from 
that  of  1859  onward.  He  declared  that  the  lavas  "  on 
Hawaii  discharge  no  steam  and  few  gases — except  the 

132 


<    -5 


Volcanoes 

air  they  inhale "  (p.  I7o).3  The  water- vapour  in  the 
clouds  formed  he  attributed  to  moisture  in  the  atmosphere, 
which  was  carried  upward  by  the  rising  column  of  hot 
air,  and  to  surface  waters  which  had  soaked  into  the 
lavas.  Green  went  even  further  than  Brun,  for,  in  regard 
to  the  great  volcanoes  of  Hawaii,  he  denied  that  the 
ascent  of  the  lava  is  due  to  pressure  of  any  kind  of  gas. 
According  to  him,  it  is  caused  by  the  subsidence  of  the 
floor  of  the  Pacific  forcing  the  fluid  matter  up  the  pipes 


FIG.  19.— OUTLINE  OF  (a)  VESUVIUS;  (6)  THE  CONE  OF  SMERO, 
JAVA;  (c)  OUTLINE  OF  KILAUEA,  HAWAII. 

of  the   volcanoes   until,   at   fairly  regular   intervals,   the 
rising  lava  overflows  in  quiet,  non-explosive  eruptions. 

The  difference  in  form  between  Kilauea  and  such  a 
normal  volcano  as  Vesuvius,  or  Smero,  in  Java,  is  shown 
by  comparing  the  accompanying  outline  of  Kilauea 
(Fig.  19,  c) — which  shows  that  it  is  a  low,  flat  dome 
having  a  shallow  caldron  at  its  blunt  summit — with  the 
photograph  and  section  (Fig.  19,  b)  of  Smero.  The 
shape  of  Kilauea  shows  that  it  has  been  formed  by 

133 


Volcanoes 

different  processes  from  those  that  build  the  Vesuvian 
type  of  volcano.  And  as  the  paucity  of  steam  in  the 
exhalations  of  the  dome-shaped  mass  of  Kilauea  is  their 
main  difference  from  those  of  conical  explosive  volcanoes 
such  as  Vesuvius,  the  characteristic  form  and  explosive- 
ness  of  the  latter  class  may  be  attributed  to  the  steam. 

That  many  eruptions  are  certainly  due  to  steam,  may 
be  illustrated  by  the  eruption  of  Tarawera  in  1886.  Tara- 
wera  is  a  flat-topped  lava  mountain  on  the  southern  side  of 
Lake  Tarawera,  in  the  volcanic  region  of  the  North  Island 
of  New  Zealand.  In  a  hollow  near  its  western  foot  was 
the  small,  hot  lake  of  Rotomahana,  on  the  bed  and  sides 
of  which  were  a  series  ,of  boiling  springs.  Some  of  these 
springs  had  deposited  the  Pink  and  White  Terraces,  which 
were  one  of  the  most  remarkable  features  of  the  wonderful 
scenery  of  New  Zealand ;  they  were,  no  doubt,  the  most 
beautiful  group  of  terraces  on  the  earth. 

This  part  of  New  Zealand  is  of  volcanic  formation,  and 
for  some  months  before  the  Tarawera  eruption  evidence 
of  more  than  usual  activity  had  been  displayed.  At  about 
i  a.m.  on  the  loth  June,  1886,  the  inhabitants  of  Wairoa, 
a  village  eight  miles  west  of  Mt.  Tarawera,  were  awakened 
by  rumbling  noises,  which  continued  until  about  2.10, 
when  they  were  shaken  by  a  severe  earthquake,  and  heard 
the  roar  of  a  great  explosion  on  Mt.  Tarawera.  The 
explosion  threw  up  a  high,  black  cloud.  At  about  2.30 
a  second  explosion  occurred,  which  sent  up  a  white  cloud 
of  steam.  Minor  eruptions  followed,  and  by  6  a.m.  the 
eruption  was  over,  though  the  area  remained  hot  and 
steaming  for  weeks.  When  the  mountain  had  cooled 
down  sufficiently  to  be  examined,  it  was  found  that  the 
two  eruptions  had  formed  a  fissure  nine  miles  in  length, 
which  had  split  Mt.  Tarawera,  and  extended  west- 
ward under  the  former  site  of  the  Pink  and  White 
Terraces.  They  and  the  hot  lake  beside  them  had 


I 


Volcanoes 

been  blown  up,  and  a  hollow  left  500  feet  deep  beneath 
them.* 

The  tribe  of  Maoris  on  the  southern   shore  of  Lake 
Tarawera  was  exterminated,  but  the  loss  of  life  was  com- 
paratively small  owing  to  the  sparseness  of  the  population. 
The  two  explosions  covered  about  eighty  square  miles  of 
country  with  scoria  three  feet  deep,  and  a  layer  of  volcanic 
dust  up  to  an  inch  or  less  in  thickness  was  spread  over 
6,000  square  miles  of  country.     Most  of  it  was  blown  by 
the  wind  to  the  east-north-east.     No  stream  of  lava  was 
discharged  by  this  eruption,  and  it  has  been  distinguished 
from  ordinary  volcanic  phenomena  as  simply  a  "  hydro- 
thermal  explosion."     It  appears,  however,  to  have  been 
a  true  volcanic   eruption.     A  fault  passes  through  this 
district,  and   cuts   across   Mt.  Tarawera.     The   eruption 
was  occasioned  by  the  ascent  of  some  molten  rock  along 
this    fissure.      This    material    first    found  an   outlet   on 
Mt.   Tarawera,   and   its   escape   thence   caused   the  first 
explosion,  and   made  the  chasm   across  that  mountain. 
The  straining  of  the  district  by  the  uprise  of  the  lava  and 
the  first  explosion  enabled  the  molten  rock  to  rise  along 
the  fissure  farther  to  the  west ;  and  twenty  minutes  after 
the  first  eruption  the  lava  must  have  entered  the  ground 
beneath  the  Pink  and  White  Terraces,  which  was  already 
sodden  with  superheated  water.     The  molten  rock  raised 
the  temperature  above  the  point  at  which  the  water  could 
be  confined  by  the  pressure,  and  its  sudden  conversion 
into  steam  formed  the  second  and  major  explosion.     The 
western  eruption  was  therefore  an  explosion  of  superheated 
water,  and  was  similar  in  nature  to  the  eruption   of  a 
geyser. 

The  explosion  of  Krakatoa,  nearly  three  years  before  that 
of  Tarawera,  illustrates  another  variation  in  the  part  played 
by  water  in  volcanic  eruptions.  Krakatoa  is  an  island  in 
the  Straits  of  Sunda,  and,  as  it  has  been  known  during 


Volcanoes 

historic  times,  it  was  only  the  stump  of  an  old  volcanic 
cone  which  had  been  blown  to  pieces  by  some  ancient 
eruption.  On  the  2oth  May,  1883,  the  mountain  suddenly 
burst  into  eruption ;  the  outbreaks  continued  for  some 
weeks,  but  declined  in  violence.  The  activity  increased 
again  in  the  middle  of  June,  and  a  series  of  moderate 
eruptions  continued  till  the  26th  August.  During  the 


FIG.  20. — KRAKATOA  ERUPTION. 

Volcanic  dust  fell  over  the  shaded  area  ;  the  outer  line  includes  the 
area  where  the  explosions  were  heard. 

night  between  the  26th  and  27th  the  eruptions  became  of 
terrific  violence;  the  adjacent  parts  of  Java  were  shaken 
by  air  blasts  from  the  explosions,  which  overthrew  a 
gasometer  at  Batavia,  one  hundred  miles  away.  The 
explosions  continued  from  about  i  p.m.  to  5  p.m.,  when 
the  noises  were  heard  all  over  Java.  The  explosions  were 
at  their  maximum  from  sunset  till  midnight.  They  threw 

136 


Volcanoes 

up  a  column  of  steam  and  dust  to  the  height  of  about 
twenty  miles.  The  greatest  explosion  occasioned  great 
waves  which  desolated  the  adjacent  coasts  and  drowned 
36,000  people.  Krakatoa  was  simply  blown  to  pieces; 
200  million  cubic  feet  of  it  were  blown  away ;  where, 
before  the  explosion,  the  land  had  stood  from  300  to 
1,400  feet  above  sea-level,  there  was  afterwards  a  hollow 
over  1,000  feet  below  sea-level.  This  hollow  was  part  of  a 
deep  rift  between  seven  and  eight  miles  long.  The  earlier 
charts  of  the  island  were  imperfect,  so  that  the  changes  in 
form  cannot  be  certainly  determined;  but  it  appears  to 
have  been  reduced  to  about  a  third  of  its  former  size.  The 
explosions  were  heard  to  the  west  on  the  island  of  Rodri- 
guez, 2,968  English  miles  from  the  volcano,  and  2,250 
miles  away  south-eastward  near  Alice  Springs  in  central 
Australia.  They  were  heard  over  a  thirteenth  of  the 
globe.  The  range  at  which  the  sounds  were  heard  is 
shown  on  Fig.  20.  The  vibration  due  to  these  explosions 
did  considerable  damage  in  Batavia,  one  hundred  miles 
distant.  The  eruption  discharged  no  lava  streams;  the 
material  was  all  thrown  forth  as  volcanic  bombs,  scoria, 
and  dust.  The  dust  fell  over  an  area  shown  by  Fig.  20. 

The  great  explosion,  according  to  Prof.  Judd,  was  due 
to  water  which,  during  a  long  period  before  the  eruption, 
had  slowly  percolated  through  the  ancient  lavas  and 
combined  with  them.  The  water  did  not  occur  in  any 
cavities  in  the  rocks  which  were  visible  by  even  the 
highest  powers  of  the  microscope.  The  water  was  in  * 
actual  combination  with  the  glass  of  the  lava.  Hence 
when  the  Krakatoan  lava  is  melted  it  swells  up  into  great 
masses  of  pumice,  as  can  be  observed  by  heating  some  of 
it  in  a  blowpipe  flame. 

The  eruption  was  not  due  to  water  finding  its  way 
down  fissures  and  coming  suddenly  into  contact  with  the 
molten  rock.  The  old  water-charged  lavas  were  remelted 

137 


Volcanoes 

and  the  water  converted  into  gases,  which  escaped  with 
explosive  violence.  To  use  Prof.  Judd's  words :  "  The 
cause  of  the  eruptive  action  was  due  to  the  disengagement 
of  volatile  substances  actually  contained  in  these  materials."5 
The  volatile  substances  were  derived  from  the  breaking  up 
of  the  imprisoned  water  by  the  intense  volcanic  heat. 

The  view  that  water  in  its  gaseous  form  plays  an 
important  part  in  volcanic  action  appears  to  be  firmly 
established.  The  water  and  steam  given  forth  during 
volcanic  eruptions  have,  moreover,  not  reached  the 
volcanic  rocks  at  or  just  before  the  eruption.  The  water 
comes  from  two  sources.  Some  of  it,  and  probably  the 
largest  part  of  it,  is  plutonic  water  which  has  arisen  from 
the  interior  of  the  earth ;  the  rest  of  the  water  has  been 
derived  from  the  sea  or  from  rain,  and  it  has  slowly 
percolated  underground,  or  was  deposited  in  the  rocks  at 
their  original  formation.  This  water  from  the  surface  must 
have  found  its  way  underground  long  before  the  volcanic 
eruption,  and  after  having  been  long  stored  up  under- 
ground, has  been  driven  off  by  the  intense  volcanic  heat.6 

Crater-like  hollows  are  formed  by  other  than  volcanic 
processes.  Thus  in  limestone  districts  great  circular  pits 
are  caused  by  the  solution  of  subterranean  masses  of  lime- 
stone forming  great  circular  caves.  The  collapse  of  the 
roof  then  forms  a  deep  pit.  A  magnificent  example  of  this 
type  of  caldron  occurs  at  Malta  ;  its  floor  is  cultivated, 
and  access  to  it  is  only  possible  by  a  path  hewn  down  one 
side  of  the  wall. 

A  more  remarkable  type  of  crater  is  that  near  the  Devil's 
Canyon  in  Arizona.  It  is  three-quarters  of  a  mile  in 
diameter,  and  the  floor  is  440  feet  deep  below  the  plain, 
but  600  feet  below  the  raised  rim  of  the  crater  (Fig.  21). 
The  rocks  around  this  pit  consist  of  a  sheet  of  sandy  lime- 
stone of  Carboniferous  age,  underlain  by  sandstone  varying 

138 


• 


Volcanoes 

from  white  to  red.  The  rocks  around  the  crater  are 
violently  tilted,  and  slope  outward,  usually  very  steeply ; 
at  one  place  the  beds  dip  towards  the  crater,  because 
they  were  thrown  vertical,  and  then  actually  overturned. 
The  rocks,  too,  are  shattered  and  crushed.  Elsewhere  in 
the  district  the  rocks  are  undisturbed,  and  there  are  no 
adjacent  volcanic  rocks  or  other  evidence  of  volcanic 
activity  in  the  district. 

Attention  was  first  called  to  this  locality  by  the  dis- 
covery of  many  meteorites,  which  are  famous,  as  they 
contain  microscopic  diamonds.  The  theory  was  suggested 
that  this  great  crater  was  made  by  a  colossal  meteorite, 


fl;*i 

s&** 

FlG.  21.— SECTION,  TO  THE  SCALE  I  INCH  IN  6oO  FEET,  ACROSS  THE 
DEVIL'S  CANYON  CRATER.    (AFTER  MERRILL.) 

5,  Sandstone  ;  L,  limestone. 

which  struck  the  earth  at  so  high  a  speed  that  it  punched 
this  big  hole  in  the  surface.  In  the  expectation  of  finding 
the  meteorite,  a  lease  of  the  ground  was  taken  as  a  mine, 
and  boreholes  were  put  down  to  the  depth  of  over  800 
feet  beneath  the  floor  of  the  crater ;  and  though  no 
meteorite  was  found,  the  sand,  in  a  layer  varying  from 
450  to  660  feet  deep,  contained  traces  of  iron  and  nickel. 
Further,  it  was  found  that  the  sand-grains  themselves  in 
the  upper  beds  showed,  upon  examination,  that  they  had 
been  crushed,  heated,  and  sometimes  almost  melted. 

An  earlier  explanation  of  this  crater  had  been  ad- 
vanced. It  was  thought  at  first  by  Prof.  W.  D.  Johnson 
and  Dr.  G.  K.  Gilbert  that  the  ground  had  been  sodden 

139 


Volcanoes 

with  steam  which  had  exploded  and  thus  produced  the 
crater.  It  would  have  been  caused  by  the  same  process  as 
that  which  formed  the  deep  hole  beneath  the  Pink  and 
White  Terraces  of  New  Zealand.  This  explanation  would 
be  consistent  with  the  outward  tilt  of  the  beds  around  the 
crater. 

The  subsequent  theory,  which  is  now  generally  advo- 
cated, is  that  the  crater  was  caused  by  the  fall  of  a  huge 
meteorite,  the  fragments  of  which  are  numerous  in  the 
district.  The  fall  of  this  huge  ball  of  nickel-iron,  probably 
500  feet  in  diameter,  would  have  pulverized  the  rocks, 
melted  some  of  them,  and  pushed  them  from  the  point  of 
impact  outwards  in  all  directions.  One  difficulty  in  this 
explanation  is  that  meteorites  often  fall  with  comparatively 
low  velocity.  For  example,  the  Hessle  meteorites  fell  on 
a  sheet  of  ice  from  which  they  rebounded  without  breaking 
it ;  and  though  meteorites  occasionally  bury  themselves  in 
the  earth,  they  are  generally  found  close  to  or  on  the  surface. 
It  may  be  that  the  Devil's  Canyon  meteorite  was  of  such 
enormous  size  that  it  would  have  been  less  checked  in  its 
fall  by  friction  with  the  atmosphere,  and  might  have 
reached  the  earth  still  travelling  at  a  high  velocity. 

Dr.  Gilbert  has  also  explained  the  ring-shaped  craters 
in  the  moon  as  formed  by  the  bombardment  of  its  surface 
by  colossal  meteorites ;  they  may,  however,  have  been 
formed  by  the  foundering  of  blocks  of  the  lunar  surface. 
They  are  probably  subsidence  caldrons ;  but  the  evidence 
of  the  half-molten  sand-grains  in  the  Devil's  Canyon  crater 
are  difficult  to  explain  on  any  other  hypothesis  than  that 
this  crater  is  the  only  known  case  on  earth  of  a  collision 
caldron. 

1  Sinter,  from  German  sintern,  to  drop ;  a  rock  precipitated  from 
material  dissolved  in  water.     It  may  be  calcareous  or  siliceous. 

2  W.  L.  Green,  "Vestiges  of  the  Molten  Globe,"  part  ii.,  1887, 
p.  162. 

140 


Volcanoes 


3  Ibid.,  part  ii.,  1887,  pp.  168-170,  270-272.  The  comparative 
steamlessness  of  the  Hawaiian  lavas  has  been  noticed  by  other 
observers,  such  as  Brigham,  in  1866. 

*  Some  standard  textbooks  report  that  the  terraces  had  been  buried 
by  scoria,  but  this  is  not  so.  The  terraces  were  blown  to  fragments, 
and  the  author  in  1904  found  pieces  of  them  three  miles  from 
their  site. 

5  Judd,  "The  Eruption  of  Krakatoa,"  Proc.  Roy.  Soc.,  1888,  p.  22. 

6  Since  this  chapter  was  written,  a  valuable  memoir  has  appeared, 
by  A.  L.  Day  and  E.  S.  Shepherd  ("  Water  and  Volcanic  Activity," 
Bull.  Geol.  Soc.  America,  December,  1913,  vol.  xxiv.,  pp.  573-606, 
Plates  XVII.-XXVII.),  in  which,  after  a  detailed  study  of  the  gases 
exhaled  from  the  Hawaiian  volcanoes,  they  refute  Brun's  conclusions 
and  show  that  water  is  abundantly  discharged.    They  announce, 
further,  the  important  discovery  that  no  argon  is  included  among 
the  volcanic  gases,  and  that  therefore  the  nitrogen  given  forth  is  not 
derived  from  the  atmosphere.     Their  observations  therefore  indicate 
that  the  water  is  in  the  main  of  plutonic  origin,  though  some  of  the 
steam  discharged  from  volcanoes  is  no  doubt  evaporated  rain-water. 


CHAPTER  IX 

HOW  MOUNTAINS  ARE  MADE 

THE  main  relief  of  the  globe  is  found  in  its  continental 
masses  which  are  separated  by  the  deep  basins  formed  by 
the  foundering  of  the  ocean  floors.  Mountains  are  the 
upstanding  ridges  and  peaks  that  rise  above  the  general 
level  of  the  land.  The  explanation  of  the  origin  of  moun- 
tain-chains is  one  of  the  fundamental  problems  of  geology. 
The  first  attempts  to  solve  this  problem  invoked  the 
action  of  the  internal  heat  of  the  earth.  The  rocks  in 
mountain-chains  are  generally  crumpled  and  folded,  as 
if  they  had  been  squeezed  into  a  smaller  space ;  and  the 
simplest  explanation  of  this  compression  is  that  the 
unshrinkable  crust  was  corrugated  owing  to  the  internal 
mass  of  the  globe  having  shrunk  as  it  slowly  cooled. 

The  same  crumpling  of  the  crust  would  be  caused  by 
its  expansion  if  it  were  being  heated  from  the  interior  of 
the  globe.  So  the  folding  of  mountain-chains  might  be 
explained  without  appeal  to  a  decrease  in  the  size  of 
the  earth.  Hence  arose  a  famous  theory  which  involved 
the  paradox  that  mountains  are  due  to  subsidence  and  not 
to  uplift.  According  to  this  view,  mountain  formation 
consists  of  four  successive  processes:  first,  a  deep  subsi- 
dence of  part  of  the  crust ;  second,  the  filling  of  the  hollow 
thus  formed  by  a  great  thickness  of  sedimentary  rock ; 
third,  the  heating  of  the  lower  layers  of  this  mass  by  the 
internal  heat  of  the  earth  ;  and  fourth,  the  buckling  of 

142 


How  Mountains  are  Made 

these  sedimentary  rocks  in  consequence  of  their  expansion 
by  the  heat. 

The  American  geologist  Dana  referred  to  the  fact  that 
many  mountain-chains  are  composed  of  continuous  series 
of  sediments,  the  total  thickness  of  which  might  amount 
to  miles,  and  yet  every  bed  in  it  had  been  laid  down 
in  shallow  water.  Hence  these  sediments  must  have 
accumulated  at  the  same  rate  as  the  sinking  of  the  ground. 
As  the  first-formed  of  these  beds  sank,  and  were  buried 
deep  beneath  the  surface,  they  must  have  become  intensely 
heated.  If  the  series  were  30,000  feet  thick,  and  the 
increase  of  temperature  be  i°  F.  for  every  fifty-three  feet 
of  descent  from  the  surface,  then  the  temperature  of  the 
rocks  at  the  bottom  of  this  area  would  be  raised  by 
566°  F.,  and  their  width  would  be  increased.  This  lateral 
expansion  could  be  most  easily  effected  by  the  bending  of 
the  sheet  until  it  became  corrugated. 

A  folded  mountain-range  would  thus  be  formed  in  con- 
sequence of  subsidence.  This  theory  was  specially 
suggested  for  the  Appalachian  Mountains ;  but  the 
principle  does  not  seem  applicable  to  the  world  as  a 
whole.  It  is  no  doubt  true  that  thick  series  of  beds 
accumulate  in  sinking  areas,  and:  such  subsidences  are 
apt  to  occur  along  weak  belts  of  the  crust,  which  most 
readily  sink  at  one  time  and  rise  at  another. 

Mountains,  however,  do  not  generally  occur  along  bands 
of  great  sedimentation.  For  example,  in  Europe  the 
series  of  sediments  in  the  Alps  is  shorter  and  thinner  than 
in  England,  and  some  of  its  thickest  beds  were  deposited 
during  the  uprise ;  and  the  Russian  plains,  which  are 
in  many  places  floored  with  thick  series  of  sediments, 
have  long  remained  undisturbed  by  earth  movements. 

It  is  true  that  some  mountain  -  chains  such  as  the 
Appalachian  Mountains  are  almost  entirely  composed  of 
sedimentary  rocks,  but  others  are  composed  of  igneous  and 

143 


How  Mountains  are  Made 

metamorphic  rocks,  and  others  of  mixtures  of  all  the  three 
chief  classes  of  rocks. 

The  arrangement  of  mountain-chains  is  independent  of 
the  distribution  of  their  rocks,  so  that  the  sedimentary 
hypothesis  is  improbable. 

Of  the  other  available  explanations,  the  most  widely 
accepted  attributes  mountains  to  the  crumpling  of  the 
crust  as  the  earth  grows  smaller.  According  to  a  homely 
illustration,  mountain-chains  are  formed  like  the  wrinkles 
of  a  tablecloth  when  it  is  pressed  into  a  smaller  width  by 
being  pushed  across  a  table.  When  an  apple  dries  the 
loss  of  water  causes  the  pulp  to  shrivel,  and  as  the  peel 
does  not  contract  to  the  same  extent,  it  is  thrown  into 
small  folds.  These  folds  are  comparable  to  the  fold 
mountain-chains  on  the  earth.  The  apple-skin  becomes 
wrinkled  all  over,  and  it  was  at  first  thought  that  the 
shrinkage  of  the  interior  of  the  earth  would  crumple  the 
whole  crust  into  a  uniformly  distributed  series  of  mountain 
chains.  According  to  the  famous  theory  of  Elie  de 
Beaumont,  the  whole  earth  was  traversed  by  a  sym- 
metrical network  of  lines,  along  which  mountain-chains 
had  been  developed. 

The  mountains  on  the  earth  are,  however,  not  evenly 
and  symmetrically  distributed.  In  an  ordinary  apple  the 
peel  is  evenly  wrinkled,  because  it  is  flexible  and  approxi- 
mately uniform  in  composition  ;  but  if  the  apple  has 
a  thickened  scar  on  the  peel,  this  part  is  not  wrinkled, 
and  the  wrinkles  are  bent  so  as  to  pass  around  it.  The 
earth  has  many  such  scarlike  areas,  which  have  stubbornly 
resisted  folding ;  but  even  allowing  for  them,  the  distribu- 
tion of  mountain-chains  is  more  restricted  than  are  the 
wrinkles  on  the  dried  apple. 

The  explanation  that  mountains  are  due  to  the  crust 
being  crumpled  so  as  to  accommodate  itself  to  the  shrink- 
age of  the  internal  mass  of  the  earth,  is  generally  based  on 

144 


How  Mountains  are  Made 

the  assumption  that  the  earth  as  a  whole  is  slowly  cooling  j1 
but  according  to  Prof.  Joly  the  earth  is  growing  hotter 
owing  to  the  influence  of  radio-active  materials  within 
it.  He  suggests  that  the  earth  is  alternately  heated 
by  radio-active  bodies  and  then  cooled  during  their  decay, 
and  that  periods  of  reheating  are  those  of  intense  volcanic 
activity. 

According  to  these  views  the  life  of  the  earth  as  a  whole 
is  due  to  radio-active  changes.  Uranium  and  the  sub- 
stances derived  from  it  would  serve  as  the  earth's  blood ; 
when  the  circulation  is  active  the  body  temperature  would 
rise  to  fever  heat,  which  might  be  relieved  by  a  rush  of 
volcanic  eruptions.  At  this  time  the  skin  may  be  stretched 
and  wrinkled  by  the  abnormal  heat ;  then  follows  a  relapse 
and  reaction,  in  which  the  temperature  will  be  subnormal, 
the  weakened  surface  would  sag  inward  on  the  exhausted 
body,  and  the  earth  would  enter  a  long  period  of  inaction, 
or,  like  the  moon,  pass  into  a  state  of  death. 

This  conception  of  the  history  of  the  earth  is  attractive, 
for  it  coincides  with  that  view  of  the  periodicity  of  the 
great  earth  movements  and  volcanic  activity,  which  is  one 
of  the  best  established  facts  in  the  history  of  the  earth. 
But  that  this  alternation  of  tumult  and  repose  is  due 
to  the  action  of  radium  is  unproved,  and  appears  im- 
probable. 

The  amount  of  radium  in  rocks  is  almost  infinitesimal ; 
it  is  measured  in  billionths  of  a  gram  (a  gram  is  15!  grains, 
or  about  -^  ounce  avoirdupois),  hence  it  is  not  surprising 
that  the  estimates  of  quantities  differ  considerably. 
According  to  the  measurements  of  Prof.  Strutt,2  the 
rocks  richest  in  radium  are  those  allied  to  granite ;  and 
among  the  igneous  rocks  those  containing  most  iron,  such 
as  basalt,  are  the  poorest  in  it.  The  igneous  rocks  are  as 
a  whole  much  richer  than  the  secondary  rocks,  among 
which  some  limestones  are  several  times  richer  than  most 

145  K 


How  Mountains  are  Made 

sandstones  and  clays.  There  are  exceptions  to  these 
rules.  Thus  among  the  igneous  rocks,  seven  normal 
granites  (excluding  special  varieties  of  granite)  gave  the 
average  of  3*7  parts  of  radium  in  a  billion  parts  of  granite 
(i.e.,  3*7  parts  in  1,000,000,000,000  parts).  In  one  of  the 
seven  granites  the  amount  was  below  2,  and  in  three 
it  was  above  4  parts  per  billion. 

Prof.  Strutt's  test  of  four  specimens  of  basalt  gave  the 
average  of  only  i  part  in  two  billion  parts  of  basalt,  so 
that  according  to  these  tests  basalt  contains  about  one- 
seventh  as  much  radium  as  granite.  But,  according  to 
Prof.  Joly's  tests,  the  basalts  are  richer  than  the  granites,3 
for  his  measurements  indicated  that  the  basic  rocks  have 
4*9  parts  per  billion,  the  granitic  rocks  4*1  parts  per 
billion,  and  recent  lavas  7*1  parts  per  billion.  Prof. 
Strutt's  test  of  a  lava  from  Vesuvius  only  gave  i'66  parts 
per  billion,  whereas  Prof.  Joly's  gave  19-2  parts  per 
billion. 

Radium  is  also  present  in  all  sedimentary  rocks  ;  some 
of  it  has  been  introduced  into  them  from  the  igneous 
rocks  from  which  their  materials  were  derived,  and  in 
some  cases  it  was  collected  from  the  sea-water.  The 
secondary  rocks,  according  to  Prof.  Joly,  contain  on  an 
average  4  parts  of  radium  in  a  billion  parts  of  rock, 
whereas  igneous  rocks  contain  an  average  of  5  parts  per 
billion ;  but  the  slowly-formed  oozes  on  the  floors  of  the 
deep  sea  are  much  richer,  for  Joly's  tests  showed  an  average 
of  7 '2  parts  of  radium  per  billion  in  oozes  composed 
largely  of  carbonate  of  lime,  27  parts  per  billion  in  the 
deep-sea  red  clays,  and  36*7  parts  per  billion  in  the  ooze 
composed  of  siliceous  organisms  (radiolarian  ooze);  and 
in  one  red  clay  the  amount  was  as  much  as  52*6  parts  per 
billion.* 

Although  the  amount  of  radium  in  rocks  is  so  small, 
it  has  been  suggested  that  the  heat  due  to  radio-activity  is 

146 


How  Mountains  are  Made 

sufficient  to  produce  Alpine  mountains.  It  is  well  known 
that  at  certain  parts  of  the  earth  the  heat  increases  down- 
ward more  rapidly  than  at  others.  Certain  mines  are 
hotter  than  others  of  the  same  depth  ;  even  one  level 
of  a  mine  may  be  hotter  in  one  place  than  in  others. 
The  irregular  distribution  of  the  heat  below  the  surface 
was  clearly  shown  in  the  Simplon  Tunnel,  where  the 
maximum  temperature,  132°  F.,  was  higher  than  had 
been  expected.  Great  attention  was  called  to  this  fact, 
owing  to  the  difficulties  thus  caused  in  the  excavation 
of  the  tunnel.  The  trouble  was  not  so  much  with  the 
actual  temperature  when  the  heat  was  dry,  for  furnace- 
men  and  people  engaged  in  cleaning  ovens  and  potters' 
furnaces  withstand  much  higher  temperatures  without 
serious  trouble.  But  in  the  Simplon  Tunnel,  as  in  the 
Comstock  Mine,  quantities  of  hot  water  flooded  the  exca- 
vations, and  scalded  the  workmen.  Prof.  Joly  has  ex- 
plained the  high  temperature  by  radio-activity,  as  the 
radium  content  of  the  rocks  was  6*3  to  7*3  parts  in  the 
billion  ;  in  the  St.  Gothard  Tunnel,  where  the  heat  was 
less,  the  rocks  ranged  in  radio-activity  from  3*4  to  77  parts 
per  billion ;  but  Prof.  Joly  concludes  that  the  average  at 
the  Simplon  was  above  that  at  the  St.  Gothard,  and  thus 
explains  the  higher  temperature  of  the  Simplon  rocks. 
Even  in  this  case  the  evidence  is  not  convincing,  as  the 
greatest  amount  of  radium  in  the  deepest-seated  Alpine 
rocks  yet  collected  is  not  particularly  high ;  but,  according 
to  Prof.  Joly,  the  influence  of  radium  might  be  sufficient 
to  cause  such  intense  overheating  of  the  area  that  the 
expansion  of  the  crust  would  cause  it  to  wrinkle  into  the 
Alpine  earth  folds. 

The  evidence  of  the  distribution  of  radium  in  the  earth 
does  not,  however,  appear  consistent  with  the  distribution 
of  mountain-chains  and  earth  movements. 

The  materials  richest  in  radium  are  those  on  the  ocean 

147 


How  Mountains  are   Made 

floors,  which,  according  to  many  geologists,  are  the  most 
stable  parts  of  the  earth's  crust.  Even  if  we  do  not  go  so 
;  far  as  those  who  hold  that  the  ocean  floors  have  been 
'permanent  throughout  geological  time,  it  must  be  ad- 
mitted that  the  great  level  plains  of  ooze  on  the  ocean 
floors,  though  the  richest  in  radium,  appear  to  be  very 
stable  at  the  "present  time.  The  most  mobile  parts  of 
the  earth's  crust  are  the  western  coasts  of  America, 
the  Japanese  archipelago,  and  the  belt  extending  from 
western  Europe  to  Malaysia  along  the  Alpine-Himalayan- 
Mountain  System,  and  its  continuation  westward  across 
the  Atlantic  to  the  West  Indies  and  eastward  through  the 
Malay  Archipelago  to  northern  Queensland  and  Mela- 
nesia. The  most  mobile  parts  of  these  areas,  as  along 
western  America,  are  off  the  coast,  in  areas  composed 
of  coarse-grained  sedimentary  deposits,  which  are  the 
poorest  of  all  terrestrial  materials  in  radium.  Thus, 
according  to  Joly,  whereas  the  deep-sea  oozes  have  from 
30  to  50  parts  of  radium  to  the  billion,  the  shallower 
calcareous  oozes  contain  only  from  6  to  8  parts  per 
billion,  and  the  coarse  shore  deposits  contain  only  3  to 
5  parts.6 

The  mobile  parts  of  the  earth's  crust  are  arranged  in 
long  continuous  bands,  and  so  far  as  the  results  given 
have  gone  it  is  the  acid  granitic  igneous  rocks  that  are 
the  richest  in  radium,  yet  these  areas  of  granite  include 
some  of  the  most  stable  on  earth. 

The  distribution  of  radium  appears  wholly  inconsistent 
with  the  distribution  of  these  mountain  belts,  which  cut 
across  areas,  quite  regardless  of  their  age  and  compo- 
sition. It  is  true  that  they  are  often  deflected  to  pass 
around  a  mass  of  hard  rock,  but  if  the  mass  cannot  be 
avoided,  the  mountain  movements  pass  through  it. 

No  theory  of  mountain  formation  is  complete  unless  it 
explains  the  distribution  of  mountain-chains,  and  the  only 

148 


How  Mountains  are  Made 

theory  that  attempts  such  an  explanation  is  that  which 
represents  the  earth's  crust  as  disturbed  by  two  series  of  * 
movements.     There  is  a  slow  heaving,  which  gently  up-  •  - 
lifts  and  lowers  wide  areas  ;  and  there  is  the  violent  com-  •  « 
pression    of    long    narrow   bands,   which   are   crumpled, 
faulted,  and  smashed,  and  these  bands  are  upraised  into 
fold  mountain-chains.     Alpine  mountains  are  due  to  the 
latter  type  of  movement,  and  in  the  Alps  great  slabs  of 
rock,  thousands  of  feet  thick  and  miles  in  width,  have 
been  thrust  forward  on  to  younger  rocks.    This  movement 
has  taken  place  along   thrust  planes,  like   those  of  the 
North- West  Highlands  of  Scotland  (see  p.  205),  but  on  an 
even  greater  scale.     Thus,  rocks  regarded  as  belonging  to  • 
southern  Switzerland  have  been  pushed  on  to  the  top  of 
the  younger  rocks  far  to  the  north.     Some  of  the  peaks  in 
the  northern  Alps  consist  of  isolated  blocks  of  old  rocks, 
which  have  no  local  roots.     They  are  known  as  "moun- 
tains without  roots,"  and  are  impressive  evidence  of  the 
colossal  extent  of  the  movements  by  which  the  rocks  of 
the  Alpine  region  have  been  rolled  into  a  jumbled  mass. 

The  possibility  of  these  great  lateral  movements  is  due 
to  the  crust  of  the  earth  consisting  of  two  layers.  The 
lower  zone  is  the  zone  of  flexure,  in  which,  owing  to  the 
great  heat,  the  rocks  are  plastic ;  hence,  like  the  flexible 
peel  of  the  apple,  the  rocks  that  have  been  long  in  this 
plastic  zone  have  been  uniformly  wrinkled,  and  the  very 
oldest  rocks  in  all  parts  of  the  world  are  almost  always 
contorted.  The  upper  zone  is  composed  largely  of  rigid 
materials,  and  though  the  crust  bends  when  considered 
on  a  large  scale,  the  bending  is  often  due  to  displacements 
along  cracks,  such  as  joints,  which  cut  through  the  rocks. 
As  the  crust  is  gradually  squeezed  into  a  smaller  area, 
owing  to  the  shrinkage  of  the  interior  of  the  earth,  instead 
of  a  uniform  wrinkling,  two  chief  movements  happen.- 
First,  wide  areas  sag  downward  where  the  crust  is  left-* 

149 


How  Mountains  are  Made 

unsupported  owing  to  the  contraction  of  the  core  of  the 
.  earth  ;  second,  these  sinking  areas  are  separated  from  the 
stationary  areas  by  great  fractures  or  folds.  The  sinking 
areas,  as  a  rule,  founder  slowly  and  imperceptibly ;  but 
their  fractured  margins  are  often  disturbed  by  sudden 
slips  along  the  fractures,  and  these  slips  are  felt  as 
earthquakes.  The  foundering  areas  are  mapped  out  for 
us  by  the  earthquake  belts  along  their  edges. 

The  sagging  of  wide  areas  of  the  rigid  outer  crust  no 
doubt  causes  heavy  pressure  on  the  plastic  layer  below; 
and  this  forces  the  plastic  material  to  flow  toward  the  less 

V    / 
V 


FIG.  22. — VOLCANIC  ERUPTIONS  DUE  TO  THE  PLASTIC  MATERIAL 
(BLACK)  DISPLACED  BY  THE  SINKING  OF  THE  FLOOR  OF  THE 
OCEAN  BASIN  RISING  UP  THE  STEP  FAULTS  (F)  AND  ESCAPING 

THROUGH   THE  VOLCANOES   (V). 

compressed  stationary  areas  (Fig.  22).  During  this  move- 
ment, the  plastic  material  passes  beneath  the  marginal 
fractures ;  and  it  may  force  its  way  to  the  surface  through 
them  and  thus  escape  in  volcanic  eruptions.  Hence  the 
chief  volcanic  belts  of  the  world  occur  beside  the  founder- 
ing areas ;  and  when  the  sinking  has  stopped,  the  volcanic 
action  also  becomes  extinct. 

Much  of  the  plastic  subterranean  material  may  flow  on 
beyond  the  channels  which  feed  the  volcano,  and,  in  this 
way,  helps  the  wide  uplift  of  the  land.  In  many  cases 
a  further  effect  has  happened,  giving  rise  to  chains  of  fold- 


How  Mountains  are  Made 

mountains.     The  land  will  have  been  left  laterally  unsup- 
ported owing  to  the  subsidence  of  the  adjacent  blocks,  and 
thus  the  upper  layers  tend  to  move  toward  the  ocean  basin, 
while  the  flow  of  the  plastic  material  from  beneath  the  sea 
toward  the  land  tends  to  drag  the  lower  parts  of  the  rigid 
crust  with  it  inland.  On  the  western  sides  of  the  continents 
these  two  forces  press  the  lower  layers  of  the  continental 
margin   in   one   direction   and  the   upper   layers   in   the 
opposite  direction.     There  will  be  a  great  deformation 
and   displacement;  on  the  western   side  of  a  continent 
raised  areas  are  thus  pressed  on  to  a  foundation  formerly 
to  the  west  of  it.     The  greatest  actual  movement  will 
probably  be  in  the  lower  layers,  as  they  are  plastic ;  but  it 
is  difficult  to  say  how  much  of  the  displacement  was  due 
to  the  actual  westward  movement  of  the  land  and  how 
much  to  the  lower  layers   flowing   eastward   under   the 
continent.     The  result  is,  however,  that  the  upper  layers 
are  forced  on  to  a   fresh   foundation;   and   during   this 
movement  the  rocks  are  faulted  and  overfolded,  and  give 
rise  to  mountain-chains  of  the  Alpine  type. 

This  same  process  explains  why  fiords  are  characteristic 
of  western  coasts.  Fiords  are  one  among  other  features 
resulting  from  the  vertical  movements  of  the  earth's 
surface ;  they  occur  where  wide  areas  of  old  rocks  have 
been  gentl>  and  slowly  upheaved.  The  upheaval  cracks 
the  crust  along  two  series  of  intersecting  lines;  the  cracks 
in  each  of  the  two  series  are  approximately  parallel,  though 
the  directions  naturally  vary  to  take  advantage  of  older 
.lines  of  weakness,  such  as  faults.  In  all  the  fiord  areas 
the  fiords  and  their  valleys  are  arranged  in  a  network ; 
while  on  the  margins  of  the  areas  the  main  fiords  and 
fiord-channels  are  usually  arranged  in  curved  concentric 
series. 

The  chief  groups  of  fiords  are  situated  around  the  polar 
areas ;  thus  in  the  Northern  Hemisphere  the  chief  fiord 


How  Mountains  are  Made 

areas  are  Alaska  and  British  Columbia,  Greenland  and 
the  islands  to  the  west  of  it,  western  Iceland,  western 
Scotland  and  Norway;  in  the  Southern  Hemisphere  the 
chief  fiords  are  in  Patagonia,  New  Zealand,  Grahamland, 
and  the  subantarctic  islands  such  as  South  Georgia  and 
Kerguelen.  The  situation  of  fiords  in  these  circumpolar 
belts  is  due  to  the  polar  areas  haying  been  most  affected 
by  widespread  slow  oscillations  due  to  the  ground  rising 
at  one  time  and  falling  at  another.  The  polar  regions  are 
more  often  affected  by  this  panting  oscillation  than  the 
1  tropical  regions.  Such  movements  are  due  to  a  deforma- 
tion of  the  earth  which  affects  the  polar  regions  more  than 
the  tropical  girdle  of  the  earth.8 

*  The  fiord  belts  are  then  more  often  situated  on  the 
western  sides  of  the  lands  than  on  the  eastern  coasts. 
This  arrangement  is  correlated  with  the  generally  greater 
elevation  of  the  western  parts  of  continents  and  larger 
lands;  it  is  exemplified  in  North  and  South  America, 
Scandinavia,  the  British  Isles,  India,  and  New  Zealand. 
It  is  apparently  due  to  the  tilting  of  the  earth  blocks  under 
the  influence  of  the  earth's  rotation.  The  great  rivers  of 
the  world  flow  eastward.  The  exceptions  are  in  the 
western  plains  of  Siberia,  north-eastern  Europe,  the  Nile, 
Congo,  and  Orange  River  in  Africa,  the  Indus,  and  the 
Murray  River  in  Australia.  The  eastward  flow  of  the 
rivers  is  due  to  the  main  land  slopes  being  eastward,  while 
the  short  steep  slopes  face  the  west.  This  general  slope  of 
the  land-surfaces  may  be  explained  by  the  influence  of  the 
earth's  rotation  on  areas  which  are  being  upraised  or 
lowered.  As  the  earth  revolves  from  west  to  east,  if 
anything  be  shot  upward  it  would  lag  behind  from  east 
to  west :  if  a  body  be  dropped  down  a  deep,  vertical  mine 
shaft,  as  it  has  on  the  surface  a  quicker  movement  to  the 
east  than  has  the  bottom  of  the  shaft,  it  will  fall  toward 
the  eastern  side  of  the  shaft.  Hence,  any  block  of  the 

152 


How  Mountains  are  Made 

earth's  crust  that  is  being  raised  will  lag  westward,  while 
a  sunken  area  will  press  eastward. 

This  influence  must,  during  the  last  uplift  of  the  Andes, 
have  pushed  the  raised  land  westward,  while  the  sunken 
areas  off  the  western  coast  of  South  America  pressed 
eastward  against  the  roots  of  the  Andes  (Fig.  23).  Both 
the  Andes  and  the  western  mountains  of  North  America 
show  many  cases  of  overthrusts  from  east  to  west.7  The 
combined  high  level  pressure  to  the  west  and  lower  level 
pressure  to  the  east  was  naturally  most  easily  relieved 
by  an  elevation  of  the  western  part  of  South  America,  and 


FIG.  23.— THE  MOVEMENTS  OF  SINKING  AND  RISING  LAND  ON  THE 
EASTERN  AND  WESTERN  COAST  OF  A  CONTINENT. 

In  A  the  land  is  sinking  on  the  western  side  of  FF,  and  rising  on  its 
eastern  side  ;  the  western  side  therefore  tends  to  press  eastward  under 
the  root  of  the  mountains. 

In  B  the  land  is  sinking  on  the  eastern  side  of  F'F',  and  it  tends  to  lag 
eastward  away  from  the  mountains. 

the  continent  was  given  a  general  slope  downward  to  the 
east.  The  same  thing  happened  in  North  America,  and 
on  most  of  the  lands  which  trend  north  and  south.  On 
the  eastern  side  of  the  continent  the  movements  were 
reversed ;  the  raised  lands  and  the  sunken  ocean  floors 
tended  to  travel  apart,  instead  of  being  forced  together; 
and  therefore  the  eastern  lands  have  sunk,  and  the  main 
ocean  deeps  lie  at  a  greater  distance  from  the  coasts.  The 
reason  why  in  Australia,  e.g.,  the  flow  of  the  rivers  appears 
reversed  is  thus  explained ;  the  main  slope  in  the  western 
plateau  of  Australia  is  south-eastward,  in  the  normal 

153 


How  Mountains  are  Made 

direction,  though  it  is  not  apparent  on  the  maps  because 
of  the  absence  of  rivers.  Formerly  Australia  must  have 
extended  far  eastward,  and  the  chief  rivers  which  flow 
down  the  eastward  slope  to  the  Pacific  would  have  formed 
one  or  more  great  rivers  with  the  normal  direction ;  the 
rivers  which  drain  the  western  face  of  the  East  Australian 
Highlands  and  unite  to  form  the  Murray  are  comparable 
to  the  short  rivers  that  flow  down  the  western  face  of  the 
great  mountain-chains. 

The  relation  of  the  great  oceanic  deeps  to  the  lands, 
show  the  influence  of  this  eastward  pressure.  It  is  because 
the  great  meridional  line  of  fractures  in  Africa  lies  along 
or  near  the  eastern  coast  that  it  occurs  as  the  Great  Rift 
Valley,  and  not  as  a  crumpled  mountain  line  like  the  Andes. 

1  This  may  be  true,  but  the  reduction  in  size  of  the  earth  might 
also  be  due  to  the  closer  packing  of  its  materials  under  gravitation. 
Geology  gives  abundant  evidence  of  the  shrinkage  of  the  earth,  but 
not  of  its  cooling.     Whether  the  contraction  be  due  to  cooling  or  to 
some  other  process  is  of  secondary  importance  to  the  geologist. 

2  The  figures  are  quoted,  as  modified  by  Eve,  to  correct  one  factor 
(Phil.  Mag.,  1907,  p.  231). 

3  J.  Joly,  "  Radioactivity  and  Geology,"  1909,  pp.  42,  43.   Prof.  Joly's 
later  results  give  lower  proportions. 

4  The  small  series  of  tests  made  include  striking  divergences  from 
the  average  results  (see,  e.g.,  Joly,  op.  cit.,  p.  52). 

6  Joly,  "  Radioactivity  and  Geology,"  1909,  p.  125.    It  should  be 
remarked  that  Prof.  Joly  explains  the  stability  of  these  radium  rich 
deep-sea  oozes  by  their  thinness,  so  that  though  they  contain  a  high  pro- 
portion of  radium,  the  total  quantity  of  radium  in  them  may  be  small. 

8  A  discussion  of  fiords  and  their  evidence  that  they  are  formed 
on  heaving  areas  of  the  earth's  crust  are  given  by  the  author  in  a 
recent  work,  "The  Nature  and  Origin  of  Fiords"  (Murray,  1913). 

7  The  earlier  uplifts  of  the  Andes  were  due  to  pressure  from 
ancient  land  to  the  west  of  the  present  coast. 


154 


CHAPTER  X 
HOW  MOUNTAINS  ARE  UPHELD 

AFTER  mountains  are  made  they  are  slowly  destroyed  by 
the  wearing  action  of  rain  and  rivers,  and  of  wind  and 
frost ;  and  mountains  may  also  disappear  by  being  engulfed 
owing  to  the  yielding  of  their  foundations.  The  upheaval 
of  most  mountain-chains  has  been  accompanied  by  the 
crumpling  of  their  rocks,  which  have  been  bent  and 
twisted  as  if  they  had  been  sheets  of  plastic  material. 
But  these  folded  rocks  are  themselves  so  hard  and  rigid 
that  any  attempt  to  bend  slabs  of  them  would  simply 
break  them  to  pieces. 

The  oldest  and  lowest  rocks  are  the  most  folded.  The 
newer  rocks  in  the  earth's  crust  are  usually  horizontal  or 
only  gently  tilted,  and  they  are  only  buckled  into  sharp 
folds  in  comparatively  narrow  bands  which  are  widely 
separated  over  the  earth.  The  Lower  Eozoic  rocks  have 
been  crumpled,  often  like  the  leather  folds  of  a  concertina, 
in  every  part  of  the  world  where  they  are  known. 
Geologists,  therefore,  early  recognized  that  the  apparently 
solid  crust  of  the  earth  has  in  some  conditions  been  so 
plastic  that  the  rocks  can  be  folded  like  sheets  of  cloth. 
It  is  well  known  that  beneath  the  earth's  surface  the 
temperature  rapidly  increases,  until  at  a  comparatively 
moderate  depth  the  heat  is  so  great  that  any  known  rocks 
would  be  melted  by  it  if  on  the  surface  of  the  earth. 
Hence  arose  the  idea  that  beneath  the  rigid  crusts  lie 
a  plastic  fluid  interior.  Physical  evidence,  however, 

155 


How  Mountains  are  Upheld 

showed  that  the  earth  is  as  rigid,  and,  according  to  some 
estimates,  is  twice  as  rigid,  as  a  ball  of  steel ;  and  it  was 
therefore  insisted  that  the  earth  must  be  solid  throughout, 
and  that  the  fluid  interior  of  geological  theory  was  impos- 
sible. The  difference,  however,  is  mainly  a  difference  of 
terms.  It  has  long  been  well  known  that  the  core  of  the 
earth  consists  of  heavier  material  than  the  crust,  and  if  the 
earth  were  composed  throughout  of  the  same  materials, 
then  the  interior  could  not  be  molten,  since  matter  is 
usually  lighter  when  liquid  than  when  in  a  solid  form. 
Nevertheless,  geologists  were  justified  in  regarding  the 
interior  as  fluid. 

-  The  essential  difference  between  a  fluid  and  a  solid  is 
that  fluids  are  plastic  and  solids  are  rigid.  But  the 
>  rigidity  of  a  material  varies  with  its  conditions.  An  ingot 
of  lead  is  sufficiently  rigid  to  resist  bending  in  the  hand  ; 
but  under  hydraulic  pressure,  even  when  quite  cold,  lead 
will  flow  through  a  jet  like  a  stream  of  water.  Lead  rods 
are  made  by  this  process,  so  that  whether  the  lead  is 
rigid  or  plastic  depends  simply  on  the  amount  of  pressure 
applied  to  it. 

The  "  New  English  Dictionary  "  defines  fluid  as  "  having 
the  property  of  flowing ;  consisting  of  particles  that  move 
freely  among  themselves,  so  as  to  give  way  before  the 
slightest  pressure."  It  defines  liquid  as  matter  in  which 
"  its  particles  move  freely  over  each  other,  so  that  its 
masses  have  no  determinate  shape."  The  "  Encyclopaedic 
Dictionary "  defines  fluid  as  "  having  the  parts  easily 
separable  ;  consisting  of  particles  which  move  and  change 
their  relative  positions  very  readily  ;  capable  of  flowing  ; 
liquid,  gaseous." 

The  rocks  on  the  earth's  surface  are  so  rigid  that  if  a 
cube  of  rock  is  placed  under  heavy  pressure  it  will  be 
crushed  to  powder,  whereas  a  cube  of  lead  would  be 
flattened  into  a  continuous  disc  owing  to  the  fact  that 

156 


How  Mountains  are  Upheld 

under  such  pressure  its  particles  move  freely  among  them- 
selves, and  have  the  property  of  flowing  ;  hence  cold  lead 
under  pressure  is  a  fluid  according  to  the  above  definitions. 
The  ordinary  rocks  on  the  earth's  surface  are  crushed 
into  powder  when  subject  to  a  weight  somewhere  betw'een 
two  and  thirty  tons  on  the  square  inch.  They  will  be 
crushed  into  fragments  by  a  pressure  of  from  two  to 
eight  tons  on  the  square  inch.  Hence  if  a  column  of 
ordinary  rocks  were  built  from  three  to  five  miles  high, 
the  base  would  be  crushed  by  the  weight  of  the  upper 
part.  Rocks  are  often  found  in  such  a  condition  of  strain 
that  they  fly  to  pieces  when  exposed  on  one  side  in  a  deep 
mine.  In  some  mining  fields  the  rocks  cut  through 
in  driving  a  mine  tunnel  suddenly  explode  owing  to  the 
strain  of  the  overlying  weight,  just  as  a  spring  may  snap 
when  overloaded.  Fragments  are  thrown  from  the  rock 
ace  in  "  rock  blasts,"  and  have  caused  many  fatal 
accidents.  Soft  rock,  like  shale,  may  be  squeezed  by  the 
pressure  of  the  overlying  rock,  so  that  it  flows  into  mine 
tunnels,  and  may  in  time  fill  them  up. 

Mt.  Everest  is  nearly  five  miles  high,  so  the  rocks  at/ 
sea-level  beneath  it  must  be  under  pressure  sufficient  to 
crush  them.  But  the  rocks  can  only  crush  if  the  fragments  * 
can  move  aside,  and  as  beneath  Mt.  Everest  they  are 
held  in  position  by  a  force  equal  to  the  weight  above 
them,  crushing  cannot  take  place.  Even  if  the  rocks 
below  the  surface  are  at  a  temperature  that  would  be  high 
enough  to  melt  them  at  the  surface,  they  cannot  melt  at 
great  depths,  because  in  melting  they  would  expand,  and 
the  weight  of  the  overlying  rock  prevents  their  expansion. 
Deeply  buried  rock  material  is  held  together  by  over- 
whelming pressure  from  all  sides.  It  therefore  remains 
solid  ;  but  if  the  rock  had  the  slightest  chance  of  movement 
— if,  for  example,  a  crack  or  cavity  were  formed  in  it — the 
space  would  be  immediately  filled  up  by  the  inflow  of  the 

157 


How  Mountains  are  Upheld 

surrounding  material.  If  a  fissure  were  suddenly  opened 
from  this  deep  layer  to  the  earth's  surface,  the  rock 
material  would  immediately  rush  up  it,  as  mud  rises 
between  two  planks  on  a  muddy  path.  As  the  rising  rock 
material  approaches  the  surface  the  pressure  on  it  is 
reduced,  so  that  it  can  expand,  and  it  thus  becomes 
molten.  The  material  at  rest  at  sea-level  beneath  Mt. 
Everest,  therefore,  resists  pressure  and  is  rigid  like  a  solid 
body,  but  its  particles  are  capable  of  flowing  over  one 
another,  and  it  is  as  ready  to  change  its  shape  as  is 
a  fluid.  The  material,  therefore,  is  in  a  condition  which 
may  be  described  as  fluidable,  for  though  so  long  as  it  is 
at  rest  it  is  solid,  it  becomes  immediately  fluid  on  the 
slightest  movement. 

The  rigid  upper  crust  of  the  earth  is  therefore  resting 
on  a  layer  which  may  be  described  as  fluidable,  and  which 
the  early  geologists  were  quite  justified  in  describing  as 
fluid.  No  doubt  some  of  them  used  the  term  carelessly, 
but  those  who  gave  most  attention  to  the  nature  of  the 
earth's  interior  and  crustal  movements  used  the  term  fluid 
in  the  sense  that  in  deep-seated  rock  the  particles  could 
move  freely  among  themselves  under  certain  conditions. 

Thus  H.  D.  Rogers  of  the  University  of  Glasgow,  in 
1858,  attributed  the  formation  of  fold-mountains  to  a  wave- 
like  pulsation  of  the  crust  of  the  earth  accompanied  by  the 
uprise  of  fluid  material  from  below  to  fill  up  any  weakened 
or  vacant  spaces.  The  wave-like  movement  of  the  crust 
he  regarded  as  possible  owing  to  the  fluid  nature  of  the 
material  below  it ;  and  calling  it  fluid  he  did  not  mean 
that  it  was  molten.  That  he  distinguished  between  fluid 
and  molten,  is  shown  by  the  following  sentence :  "  This 
oscillating  movement  in  the  fluid  mass  beneath  would 
communicate  a  series  of  temporary  flexures  to  the  over- 
lying crust,  and  these  flexures  would  be  rendered  permanent 
(or  keyed  into  the  forms  they  present)  by  the  intrusion  of 

158 


How  Mountains  are  Upheld 

molten  matter"  (p.  gn).1  He  therefore  clearly  dis- 
tinguished between  a  molten  material  like  a  lava  and 
the  fluid  interior  of  the  earth,  which  is  only  fluid  in  the 
same  sense  that  lead  is  fluid  in  a  hydraulic  press. 

The  view  that  the  earth  has  a  rigid  crust  resting  on 
a  fluidable  foundation  appears  to  be  faced  by  one  serious 
difficulty.  The  Himalaya  rise  five  miles  above  the  sea- 
surface,  while  750  miles  to  the  south  the  floor  of  the 
Indian  Ocean  lies  two  miles  below  sea-level.  Why  do 
not  the  Himalaya  and  other  high  mountains  sink  into  this 
fluid  interior  until  the  surface  of  the  earth  be  reduced 
to  a  uniform  level  ?  If  the  earth  were  composed  of  homo- 
geneous material  it  is  probable  that  its  level  would  be 
nearly  uniform.  For  all  the  agents  of  denudation  are 
slowly  wearing  away  the  raised  masses  and  depositing 
their  material  beneath  the  sea.  In  time  the  whole  surface 
of  the  earth  would  be  reduced  to  low  plains  but  little  above 
sea-level  were  not  earth  movements  continually  re-elevating 
the  continents. 

The  effect  of  earth  movements  is  therefore  to  uplift  the 
lands  instead  of  reducing  the  whole  earth  to  one  uniform 
level,  as  might  be  expected  on  the  view  that  the  interior  is 
plastic.  The  maintenance  of  mountains,  according  to  one 
school  of  geology,  is  due  to  the  rigidity  of  the  earth's 
crust,  just  as  a  railway  embankment  built  over  a  tract 
of  firm  land  is  upheld  by  the  strength  of  the  ground 
beneath  it.  But  if  the  embankment  were  placed  across 
a  peat  moss,  it  would  sink  in  owing  to  the  fluidity  of  its 
foundations;  and,  according  to  another  school  of  geolo- 
gists, the  earth's  crust  is  so  plastic  that,  like  the  peat 
moss,  the  surface  sinks  if  any  extra  heavy  load  be  placed 
upon  it. 

Geologists  were  driven  to  this  at  first  sight  incredible 
conclusion  by  many  observations.  The  British  Coal 
Measures  consist  of  a  series  of  sandstones,  clays,  coal 

159 


How  Mountains  are  Upheld 

seams,  and  occasional  beds  of  limestone,  which  are 
together  from  8,000  to  12,000  feet  in  thickness ;  yet  all  the 
members  of  this  thick  series  were  laid  down  at  about 
sea-level.  Throughout  the  formation  of  the  Coal  Measures 
the  ground  must  have  been  sinking  at  the  same  average 
rate  as  the  deposition  of  the  rocks.  Occasionally  the  land 
sank  a  little  faster,  and  it  was  then  flooded  by  the  sea.  A 
thin  layer  of  limestone  or  a  bed  of  shale  was  formed,  and 
the  fossils  in  it  show  that  the  sea  was  shallow,  and  that  the 
deposit  was  being  laid  close  to  the  shore.  Sometimes  the 
beds  were  laid  down  more  quickly  than  the  subsidence ; 
and  then  the  sea  was  kept  back,  and  the  land  was  built  up 
into  a  coastal  plain,  on  which  grew  the  dense  forests  that 
formed  the  coal  seams.  Yet  the  occasional  recurrence  of 
seams  of  limestone  show  that  the  level  of  the  land  never 
rose  much  above  that  of  the  sea. 

-  The  laying  down  of  the  rock  material  therefore  took 
place  at  the  same  rate  as  the  sinking  of  the  ground.  As 
these  two  processes  continued  at  the  same  rate  through  so 
long  a  time,  and  this  remarkable  coincidence  has  happened 
in  so  many  parts  of  the  world,  and  so  often  in  geological 
time,  that  the  conclusion  appears  inevitable  that  the  two 
processes  are  directly  connected.  Sedimentation  and 
subsidence  have  often  been  equal  because  the  sedimenta- 
tion caused  the  subsidence.  The  load  of  fresh  sediment 
forced  down  the  ground  to  a  depth  corresponding  to  its 
weight,  so  that  the  locality  was  ready  to  receive  a  fresh 
layer  of  material  laid  down  under  precisely  the  same 
geographical  conditions. 

If  two  piles  of  wood,  both  containing,  say,  three  pieces 
each  an  inch  thick,  be  placed  side  by  side  in  a  tank 
of  water,  the  top  of  the  two  floating  piles  will  be  at  the 
same  level  (Fig.  24).  If  now  a  board  be  transferred  from 
one  pile  to  the  other,  the  side  from  which  the  board  has 
been  removed  will  rise,  and  the  side  to  which  it  is  added 

160 


How  Mountains  are  Upheld 

will  sink,  so  that  the  level  of  the  two  surfaces  is  again 
about  the  same.  The  transference  of  the  load  from 
the  one  pile  to  the  other  will  have  caused  the  uprise  of 
the  one  and  the  subsidence  of  the  other.  Similarly  it  is 
believed  that  the  removal  of  a  load  of  rock  from  a  land 
which  is  undergoing  denudation,  and  its  deposition  on  the 
adjacent  sea-floor,  may  cause  the  uprise  of  the  land  as  its 
load  is  lightened,  and  the  sinking  of  the  sea-floor  on  which 
the  extra  load  has  been  placed.  If  one  pile  contain 
heavier  material  than  the  other,  then  its  surface  will  be 
lower  than  that  of  the  lighter  pile. 

According  to  this  view  all  parts  of  the  earth's  crust  are 
floating  on  a  fluid  layer,  and  their  level  is  determined  by 


FIG.  24.— ISOSTATIC  EQUILIBRIUM. 

The  transfer  of  the  slab  No.  i  from  the  left-hand  to  the  right-hand  pile 
causes  the  uprise  of  the  former  and  sinking  of  the  latter. 

their  weight.  This  assumed  balance  of  different  areas  of 
the  earth's  crust  was  called  by  Button  in  1889  "  isostasy," 
and  all  parts  of  the  earth's  surface  are  said  to  stand  in 
isostatic  equilibrium. 

Now  this  principle  would  seem  to  be  opposed  to  the 
existence  of  mountain-chains,  but  it  is  the  evidence  of 
the  mountains  which  really  confirms  it.  During  the  survey 
of  India  it  was  realized  that  the  results  might  be  incorrect 
owing  to  the  attraction  of  the  Himalaya.  Precise  levelling 
depends  upon  determining  the  exact  position  of  the  vertical ; 
this  may  be  found  with  a  plumb-line.  But  if  a  plumb-line 
be  hung  beside  a  huge  mass  of  material,  such  as  a  moun- 
tain, it  will  be  drawn  toward  the  mountain,  and  thus  not 

161  L 


How  Mountains  are  Upheld 

hang  truly  vertical.  Careful  observations  were  made  to 
determine  the  exact  extent  by  which  the  Himalaya  attracted 
the  plumb-line,  and  the  amount  was  found  to  be  much 
less  than  was  expected  from  the  calculations.  The  great 
mass  of  the  Himalaya  above  sea-level  is  compensated  by 
the  existence  of  a  deficiency  of  material  underground. 
This  deficiency  or  lightness  of  the  ground  below  the 
Himalaya  is  known  as  its  isostatic  compensation  ;  and, 
according  to  recent  calculations,  this  compensation  is 
effected  within  a  layer  of  the  earth's  crust  about  eighty 
miles  thick.  Observations  have  therefore  shown  that  the 
weight  of  the  column  of  material  extending  from  the 
summits  of  the  Himalaya  to  the  depth  of  eighty-five  miles 
below  them  is  the  same  weight  as  a  column  of  the  same  area 
eighty  miles  deep  below  the  mouth  of  the  Ganges,  and  as 
one  seventy-eight  miles  deep  below  the  Bay  of  Bengal. 
Each  of  these  three  areas,  in  spite  of  their  difference  of 
elevation,  weigh  equally  on  the  interior  of  the  earth.  They 
are  therefore  in  isostatic  equilibrium.  In  course  of  time 
the  materials  of  the  Himalaya  and  plateau  of  Thibet  are 
carried  down  by  the  rivers  and  spread  over  the  Ganges 
plain  or  floor  of  the  Indian  Ocean.  This  transference  of 
load  is  followed  by  further  movements,  which  tend  to 
maintain  the  present  difference  of  level. 

This  doctrine  of  isostasy  has  been  repeatedly  reaffirmed 
and  denied ;  and  some  geologists  adopt  a  compromise  and 
hold  that  the  continents  are  upheld  by  isostasy,  while  the 
mountain-chains  are  upheld  like  railway  embankments  by 
the  rigidity  of  the  underlying  crust. 

•  The  theory  of  isostasy  has  been  recently  tested  by  two 
very  careful  series  of  measurements.  If  the  theory  be 
true,  then  the  material  beneath  the  ocean  floors  should 
be  heavier  than  the  material  forming  the  continents.  Dr. 
Hecker  therefore  prepared  an  apparatus  by  which  the 
weight  of  the  rocks  below  the  oceans  could  be  determined 

162 


How  Mountains  are  Upheld 

at  sea.  His  method  depended  on  comparing  the  pressure 
of  the  air  as  shown  by  a  mercurial  barometer,  the  height 
of  which  is  affected  by  the  attraction  of  the  rocks  beneath 
the  ocean,  with  the  pressure  as  determined  by  the  boiling- 
point  of  water.  He  travelled  with  his  apparatus  from  the 
north  coast  of  Germany  through  the  English  Channel, 
Mediterranean,  and  Red  Sea  to  Sydney,  whence  he  crossed 
to  San  Francisco  and  returned  across  the  North  Pacific  to 
Japan.  His  observations  show  that  the  depth  of  the  oceans 
varies  with  the  weight  of  the  rocks  beneath  them.  He  thus 
confirmed  the  view  that  the  depth  of  the  ocean  is  controlled 
by  isostatic  influences. 

Dr.  Hecker  has  therefore  tested  the  question  for  the 
ocean  basins,  and  Mr.  J.  F.  Hayford  has  investigated  the 
connection  between  the  level  of  the  land  and  its  weight 
by  an  elaborate  survey  across  the  whole  width  of  the 
United  States.  His  results  have  shown  that  the  varia- 
tions in  height  agree  with  the  isostatic  principle.  The 
great  elevations  are  compensated  by  the  lightness  of  the 
materials  beneath  them. 

The  earth's  crust,  in  fact,  may  be  compared  to  a  sheet 
of  soft  material  which  was  once  uniform  in  level.  Where 
the  surface  has  been  pressed  downward,  the  material  below 
it  is  denser,  as  if  it  had  been  compressed ;  where  the  surface 
has  been  highly  raised,  the  uplift  has  permitted  the  expan- 
sion of  the  material,  so  that  it  is  now  looser  and  lighter. 

The  isostatic  compensation  is  of  course  not  complete, 
for  the  crust  has  unquestionably  considerable  rigidity; 
but,  according  to  Hayford's  work,  the  isostatic  agreement 
in  the  United  States  is  remarkably  close.  He  has  deter- 
mined eleven  areas,  as  shown  on  the  accompanying  sketch 
map2  (Fig.  25),  in  which  the  weight  of  the  material  is  in 
excess,  and  five  in  which  the  material  is  lighter  than  it 
would  be  if  the  compensation  were  perfect.  Hence  the 
level  of  the  land  is  sometimes  higher  and  sometimes  lower 

163 


How  Mountains  are  Upheld 

than  it  would  be  if  the  isostatic  compensation  were  perfect. 
But  these  areas  are  small  in  comparison  with  those  where 
the  compensation  is  complete.  If  the  same  be  true  of 
other  parts  of  the  world,  then  the  continents  are  floating 
because  they  are  composed  of  light  material,  and  the 
ocean  floors  are  depressed  because  they  are  composed  of 
dense  materials. 

The  principle  of  isostasy  is  necessarily  a  hypothesis  to 
explain  certain  facts,  but,  according  to  Hayford,  "  For  the 


FIG.  25.  —  MAP  SHOWING  THE  DISTRIBUTION  OF  AREAS  IN  THE 
UNITED  STATES  WHERE  THE  SURFACE  is  NOT  AT  THE  LEVEL  OF 
ISOSTATIC  EQUILIBRIUM.  (AFTER  HAYFORD.) 

Dotted  areas  are  those  where  the  density  is  in  excess,  and  the  areas 
shaded  by  lines  those  where  the  density  is  deficient. 

United  States  and  adjacent  areas,  it  is  certain  that  the 
assumption  that  the  condition  called  '  isostasy '  exists,  is 
a  much  closer  approximation  to  the  truth  than  the 
assumption  that  it  does  not  exist"  (p.  175) .3  He 
further  declares  that  "  the  present  close  approach  to  per- 
fect isostatic  compensation  in  the  United  States  and 
adjacent  areas  is  a  proof  that  the  outer  portion  of  this 
part  of  the  earth  has  a  small  effective  rigidity.  The 

164 


How  Mountains  are  Upheld 

departures  from  perfect  isostatic  compensation  are  a 
measure  of  the  effective  rigidity  of  the  material  involved 
in  isostatic  readjustment "  (p.  ij6).3 

"  The  two  paragraphs  which  immediately  precede  this," 
he  concludes,  "  are  combined  in  the  statement  in  the 
following  two  sentences :  '  For  the  United  States  and 
adjacent  areas  the  assumption  of  extreme  rigidity  is  far 
from  the  truth.  The  United  States  is  not  maintained  in 
its  position  above  sea-level  by  the  rigidity  of  the  earth, 
but  is,  in  the  main,  buoyed  up,  floated,  because  it  is  com- 
posed of  material  of  deficient  density  '  "  (p.  176)  .3 

1  H.  D.  Rogers,  "The  Geology  of  Pennsylvania:  A  Government 
Survey,  with  a  General  View  of  the  Geology  of  the  United  States," 
Part  II.,  1858,  vol.  ii.,  p.  911. 

2  J.  F.  Hayford,  "The  Figure  of  the  Earth  and  Isostasy  from 
Measurements  in  the  United  States,"  Department  of  Commerce  and 
Labour,  Coast  and  Geodetic  Survey,  1909. 

3  J.  F.  Hayford,  "Supplementary  Investigation,"  Department  of 
Commerce  and  Labour,  Coast  and  Geodetic  Survey,  1910,  pp.  175, 
176. 


165 


PART  III 
HISTORICAL  GEOLOGY 

CHAPTER  XI 

THE  AGE  OF  THE  EARTH 

"  OF  old  hast  Thou  laid  the  foundations  of  the  earth," 
wrote  the  Psalmist ;  and  many  men  of  science  have  en- 
deavoured to  improve  that  statement  by  discovering  how 
old  those  foundations  are.  There  have  been  three  chief 
conflicting  views  about  the  age  of  the  earth — one  assigning 
it  a  duration  of  a  few  thousand  years,  another  a  few  tens 
of  millions  of  years,  and  a  third  regarding  the  earth  of 
inconceivable  antiquity.  Before  considering  the  history 
of  the  earth  it  is  advisable  to  determine  what  length  of 
time  is  available. 

The  study  of  geology  at  once  revealed  convincing 
evidence  that  the  earth  is  immeasurably  older  than  the 
limit  based  on  a  literal  interpretation  of  the  Bible,  or  even 
than  the  longer  allowances  of  time  made  by  the  religions 
of  the  Far  East,  which  in  this  respect  were  more  generous 
than  those  of  the  Near  East.  Geologists  quietly  assumed 
whatever  time  was  necessary  to  explain  the  geologic  facts  ; 
and  some  of  them  founded  conclusions,  which  are  possibly 
correct,  on  exaggerated  estimates  of  the  length  of  some 
chapters  in  the  earth's  history.  Geographical  agents, 
such  as  wind  and  water,  are  working  in  parts  of  England 
so  slowly  that  if  they  had  always  worked  at  their  present 
rate  the  time  required  to  give  the  country  its  present  form 

166 


The  Age  of  the  Earth 

would  be  simply  prodigious.  Hence  the  earlier  geologists 
called  in  the  aid  of  catastrophes  to  rend  the  hills  and 
flood  the  lowlands  ;  but  as  catastrophes  fell  into  discredit 
there  came  estimates  demanding  for  the  earth  an  incon- 
ceivable antiquity.  To  quote  one  famous  estimate,  Darwin 
pointed  out  that  the  streams  of  Kent  are  now  enlarging 
their  valleys  so  slowly  that  the  erosion  of  these  valleys 
would  alone  require  300  million  years.  If  it  takes  sc 
long  to  excavate  a  valley,  the  deposition  of  sheets  ol 
rock  must  also  be  slow,  and  the  time  required  for  the 
formation  of  the  rocks  found  in  south-eastern  England, 
their  folding,  the  wearing  down  of  the  upraised  land  into 
plains,  and  the  excavation  of  the  successive  systems  of 
valleys,  would  have  to  be  reckoned  in  thousands  of  millions 
of  years ;  and  this  amount  would  only  include  a  small  part 
of  the  earth's  history. 

Darwin's  estimate  was  no  doubt  exaggerated,  because 
it  did  not  allow  for  the  greater  rate  at  which  rivers  excavate 
in  a  highland  country.  When  a  country  is  lifted  high 
above  the  sea  as  a  plateau,  it  is  very  easily  gnawed  away 
by  the  action  of  rivers,  rain,  wind,  landslips,  and  other 
natural  agents.  The  rivers  quickly  cut  deep  valleys,  and 
as  the  water  rushes  down  these  with  torrential  violence, 
the  current  sweeps  down  large  rocks  and  innumerable 
pebbles,  which  wear  away  the  river-bed,  so  that  the  valley 
is  rapidly  deepened  into  a  gorge.  The  rain  and  wind 
attack  the  banks  and  wash  them  backward,  so  that  the 
gorge  is  widened  into  an  open  valley ;  this  process  is 
greatly  aided  by  huge  rock-falls  and  landslips.  The  fallen 
masses  often  block  the  stream  and  form  lakes  above  these 
destructions,  and  should  the  weight  of  the  water  burst  the 
barrier,  the  lake  is  discharged  by  a  sudden  flood,  which  has 
enormous  destructive  power.  Steep  hillsides  are  shattered 
by  frost,  and  the  material  creeps  slowly  down  the  valley 
under  the  influence  of  rain,  wind,  and  daily  variations  of 


The  Age  of  the  Earth 

temperature.  A  country  under  such  conditions  is  said  to 
have  a  young  topography,  and  it  is  in  a  very  unstable  con- 
dition. Such  a  country  appears  to  be  almost  falling  to 
pieces,  and  stream  erosion  is  very  rapid. 

In  time,  however,  the  country  is  worn  down  so  that  the 
hills  are  rounded,  the  slopes  are  gentle,  the  sea-cliffs  and 
inland  precipices  have  disappeared,  the  rivers  have  slow 
currents,  and  their  courses  are  unbroken  by  waterfalls  or 
rapids.  The  country  is  then  said  to  have  an  old  topog- 
raphy, and  stream  erosion  may  be  very  slow  or  have  even 
ceased  altogether  except  during  times  of  unusual  rain  and 
floods.  When  the  south-east  of  England  was  uplifted 
into  a  plateau,  the  streams  had  such  a  steep  slope  from 
their  sources  to  the  sea  that  they  had  great  excavating 
power.  They  would  have  plunged  over  waterfalls  and 
rushed  down  cascades,  and  their  torrential  waters  would 
soon  have  cut  deep  gorges  through  the  soft  rocks ;  and  the 
fall  of  the  banks  would  have  enlarged  the  gorges  into  wide 
valleys.  The  whole  country  would  have  been  in  an  un- 
stable condition ;  and  the  excavation  of  valleys  would 
have  proceeded  much  faster  than  it  does  now  that  the 
hills  have  been  worn  down  into  gentle  slopes,  and  the 
country  has  reached  a  condition  of  maturity.  The  rate 
of  valley  enlargement  in  an  old  and  stable  country  is  no 
use  as  a  measure  of  the  rate  in  a  young  and  unstable 
country.  For  a  time  comes  when  a  district  has  been  so 
reduced  by  denudation  that  the  rivers  have  no  further 
powers  of  erosion  ;  the  rate  of  enlargement  of  the  valleys 
is  then  reduced  to  nothing. 

To  take  the  rate  of  valley  excavation  in  an  old  and 
stable  country  as  a  universal  time  rate  for  that  operation 
would  give  results  as  exaggerated  as  if  an  estimate  of  the 
length  of  time  taken  to  carve  a  statue  were  based  on  the 
rate  of  the  last  fine  touches,  and  ignored  the  rapid  rough- 
hewing  of  the  first  stages. 

168 


The  Age  of  the  Earth 

Nevertheless,  even  allowing  for  the  more  rapid  destruc- 
tion of  lands  under  special  geographical  conditions, 
geologists  are  bound  to  assume  for  the  earth  an  age  that 
can  only  be  numbered  by  very  many  millions  of  years. 
And  the  attempt  to  determine  the  number  of  millions  has 
given  rise  to  long  controversy. 

The  first  modern  attempt  to  determine  the  age  of  the 
earth  was  by  John  Phillips,  who,  in  1860,  introduced  the 
use  of  various  geological  processes  as  time  clocks.  The 
method  that  has  been  most  frequently  employed  is  to 
estimate  the  time  required  for  the  deposition  of  one  foot 
of  sand,  clay,  and  shell  beds  on  a  sea-floor,  then  to  calcu- 
late the  total  thickness  of  the  rocks  in  the  earth's  crust ; 
and  this  thickness  multiplied  by  the  average  rate  of 
accumulation  was  claimed  to  give  the  length  of  time 
required  for  the  deposition  of  the  whole  series  of  sedi- 
mentary rocks. 

The  estimates  based  on  this  method  have  given  the 
most  variable  results.  A  paper  was  once  read  at  a 
geological  society  claiming  that  the  deposition  of  the 
formation  in  Suffolk  known  as  the  Red  Crag  had  taken 
3  million  years.  In  the  discussion  one  critic  is  said 
to  have  quoted  various  statistics  as  to  the  accumulation 
of  beds  of  sand  along  the  shore,  and  to  have  claimed 
therefrom  that  the  whole  Red  Crag  had  been  deposited  in 
a  fortnight.  The  truth  is  doubtless  somewhere  between 
the  fortnight  and  the  3  million  years,  but  which 
estimate  is  nearer  the  truth  is  doubtful.  The  estimates 
offered  as  to  the  total  length  of  time  represented  by  the 
sedimentary  rocks  have  also  varied  between  similar 
extremes.  The  nearest  modern  approach  to  the  Bible 
limit  has  come  from  Chicago,  where  the  system  of 
"  speeding  up  "  so  influenced  the  late  Prof.  A.  Winchell, 
that  from  a  table  of  ratios  of  estimated  ages  of  the 
Falls  of  Niagara,  the  Falls  of  St.  Anthony  at  Minneapolis, 

169 


The  Age  of  the  Earth 

the  Mississippi  delta  and  the  bluffs  beside  Lake  Michigan, 
he  concluded  "  the  whole  incrusted  age  of  the  world 
deduced  from  the  table  of  ratios  would  be  3  million  years."  1 
He  insisted  that  "  geological  history  has  limits  far  within 
the  wild  conceptions  of  a  certain  class  of  geologists." 
Four  years  later  the  other  extreme  in  the  estimates  of 
that  period  was  reached  in  the  more  leisurely  atmosphere 
of  Washington,  where  McGee  calculated  the  age  of  the 
earth  as  6,000  million  years.2 

Sir  Archibald  Geikie  in  1892  held  that  the  rate  at  which 
the  land  is  denuded  varies  from  one  foot  in  730  years  to 
one  foot  in  6,800  years ;  and  the  denudation  of  the  land 
supplies  a  measure  of  the  rate  of  deposition  of  rocks,  for 
the  materials  of  the  new  rocks  are  derived  from  the 
wearing  down  of  the  land.  If  we  accept  the  stratified 
rocks  as  having  a  total  thickness  of  100,000  feet,  then,  he 
argues,  if  they  were  laid  down  at  the  most  rapid  recorded 
rate  of  denudation  they  would  require  a  period  of 
73  million  years,  and  if  at  the  slowest  rate  of  denudation 
a  period  of  not  less  than  680  million  years,  and  he  took 
400  million  years  as  a  probable  time-limit  for  this  process. 

It  is  extremely  difficult  to  form  a  reliable  estimate 
of  the  rate  at  which  rocks  are  being  deposited.  The 
estimates  vary  from  one  foot  in  a  century  to  a  foot  in 
8,000  years  (Haughton).  The  former  estimates  may  be 
true  for  areas  of  rapid  deposition  in  estuaries  and  harbours. 
Silt  has  accumulated  at  one  place  in  Falmouth  Harbour 
to  the  depth  of  forty- two  feet  in  150  years ;  the  Nile 
has  deposited  delta  material  at  the  rate  of  one  foot  in 
320  years.  But  such  rates  are  exceptional ;  over  the  deep 
ocean  floors  the  deposition  of  material  must  be  so  slow 
that  the  formation  of  an  inch  of  sediment  must  require 
several  thousands  of  years.  The  deep-sea  deposits,  how- 
ever, are  only  exceptionally  found  on  land,  and  the  rocks 
which  build  up  the  main  masses  of  the  continents  are 

170 


The  Age  of  the  Earth 

those  which  have  been  laid  down  as  sheets  of  sand,  clay, 
and  limestone  in  comparatively  shallow  seas.  And  there 
are  no  reliable  direct  data  as  to  the  average  rate  at  which 
these  deposits  have  been  laid  down. 

The  chief  estimates  have  been  based  on  indirect  evi- 
dence. The  total  quantity  of  material  carried  to  sea 
every  year  by  all  the  rivers  of  the  earth  and  of  that  broken 
from  the  coast  is  estimated,  and  this  amount  is  divided 
by  the  area  in  which  the  deposits  are  being  laid  down  ; 
it  has  been  thence  deduced  that  the  total  amount  of 
material  carried  to  the  sea  annually,  including  both  the 
particles  carried  bodily  along  as  sand  and  mud  and  that 
dissolved  in  the  water,  is  some  9,000  million  tons.  This 
estimate  is  based  on  most  uncertain  data.  Careful 
measurements  have  been  made  for  a  few  rivers,  such  as 
the  Mississippi,  Rhine,  and  Po,  which  are  so  powerful 
and  destructive  that  careful  measurements  are  required 
for  the  safety  of  the  lands  beside  them.  There  are  no 
such  data  for  large  parts  of  the  earth's  surface,  and  the 
quantities  of  material  carried  to  the  sea  by  rivers  varies 
enormously.  The  Mississippi  drains  a  basin  but  slightly 
larger  than  that  of  the  Nile,  yet  it  carries  down  about 
six  times  as  much  material. 

The  most  careful  estimate  of  the  amount  of  material 
carried  to  the  sea  by  rivers  has  been  compiled  for  the 
United  States  (Dole  and  Stabler) ;  and  its  amount  is 
probably  exaggerated  owing  to  the  especial  activity  of  the 
Mississippi,  which  bears  to  the  sea  nearly  two-thirds  of 
the  total,  though  the  area  of  its  basin  is  not  much  over 
one-third.3 

The  contribution  of  sediment  to  the  sea  by  the  rivers 
may  be  easily  over-estimated,  and  there  are  no  possible 
data  for  the  amount  carried  to  the  sea  by  the  Arctic  and 
Antarctic  glaciers,  and  no  reliable  evidence  as  to  the  rate 
at  which  the  sea  is  removing  the  land.  An  estimate  of 

171 


The  Age  of  the  Earth 

700  million  tons  a  year  has  been  formed  by  assuming 
that  the  average  rate  of  marine  erosion  is  one  foot  a 
century,  that  the  coast  lines  of  the  world  are  125,000  miles 
long,  and  that  the  average  height  of  the  shore  cliffs  is 
150  feet.  Such  cliffs  appear  to  the  author  to  be  the 
exception  rather  than  the  rule;  where  cliffs  are  so  high 
they  usually  consist  of  hard  rocks,  of  which  the  rate  of 
denudation  is  generally  much  less  than  a  foot  a  century. 
The  soft  eastern  coasts  of  England  are  subjected  to  scour 
by  a  strong  tide,  and  it  is  armed  with  an  inexhaustible 
supply  of  flint  pebbles,  which  are  exceptionally  good 
grinding  material.  So  these  coasts  have  been  in  places 
worn  away  with  deplorable  rapidity.  Yet  it  appears  prob- 
able that  for  the  British  Isles  as  a  whole  there  is  no  loss 
at  all,  for  the  advance  of  the  land  in  some  places  is  equal 
to  its  recession  elsewhere. 

Along  the  western  coast  of  Scotland,  although  exposed 
to  the  full  fury  of  the  North  Atlantic,  the  wearing  back  of 
the  land  by  the  sea  during  historic  times  has  been  in- 
appreciable, owing  to  the  resistance  of  its  hard,  ice- 
smoothed  rocks.  And  along  all  coasts  fringed  by  wide 
beaches  the  sea  churns  up  the  loose  shore  material, 
removing  it  from  one  place  and  redepositing  it  at  another, 
without  attacking  the  solid  rocks  at  all. 

The  quantity  of  material  removed  from  the  land  at 
present  is  still  so  uncertain  that  it  gives  no  reliable 
estimate  of  the  rate  of  rock  formation. 

Another  correction  is  to  be  supplied  for  the  shrinkage 
of  rocks,  which  appears  to  go  on  for  prolonged  periods 
after  their  formation.  According  to  the  last  paper  by 
Dr.  Sorby,  ordinary  shales  are  now  only  one-sixth  of  their 
original  thickness.  Chalk  is  as  much  as  a  half,  while 
some  rocks  are  only  one-tenth  of  their  original  thickness. 
Since  chalk  is  regarded  as  having  undergone  an  exception- 
ally small  shrinkage,  we  may  assume  that  rocks  have 

172 


The  Age  of  the  Earth 

shrunk  on  the  average  to  one-fourth  or  one-fifth  of  their 
original  volume;  hence  the  period  required  for  the  for- 
mation of  the  stratified  rocks,  judged  by  the  rate  of  accu- 
mulation of  existing  sediments,  must  be  multiplied  four 
or  five  times,  owing  to  the  greater  compactness  of  the 
old  rocks. 

Even  if  we  knew  the  rates  of  denudation  and  of  the 
formation  of  rocks  as  they  now  exist,  these  data  would 
give  no  satisfactory  evidence  of  the  age  of  the  earth  unless 
the  rates  had  been  uniform  throughout  geologic  time,  and 
that  is  most  improbable.  We  are  living  in  a  period  in 
which  the  work  of  rivers  in  wearing  away  the  land  is 
exceptionally  rapid. 

Many  of  the  best-known  areas  in  the  Northern  Hemi- 
sphere and  the  very  areas  for  which  observation  on  river 
action  have  been  most  careful  and  most  numerous  were 
recently  affected  by  ice-sheets  and  uplift.  The  lowlands 
have  been  covered  by  glaciers,  and  many  valleys  have 
been  filled  by  debris  deposited  by  glaciers,  or  by  floods 
of  water  from  the  melting  ice.  Such  materials  are  very 
easily  washed  away,  and  as  vast  quantities  have  been 
dropped  across  the  valleys,  the  rivers  are  forced  to 
attack  them.  Rivers  which  flow  through  areas  covered 
by  glacial  deposits  are  therefore  likely  to  remove  excep- 
tionally large  amounts  of  rock  debris. 

Moreover,  the  earth  has  in  recent  geological  times 
undergone  one  of  its  periods  of  active  earth  movements 
and  mountain  formation.  It  has  often  been  maintained, 
for  example,  that  the  Andes  and  the  Himalaya  are  still 
in  process  of  uplift.  There  can  be  no  doubt  that  large 
parts  of  north-western  Europe  were  uplifted  into  a  plateau 
in  Pliocene  times,  and  that  most  of  Scandinavia  is  still 
undergoing  uplift,  and  that  western  North  and  South 
America  and  New  Zealand  are  still  undergoing  great  oscil- 
lations of  level.  Such  earth  movements,  as  often  as  they 

173 


The  Age  of  the  Earth 

occur,  leave  the  land  in  an  unstable  condition,  with  a 
young  topography  and  unusually  active  rivers. 

In  other  parts  of  the  earth's  history  there  were  prob- 
ably long  intervals  when  the  crust  was  quiet  and  undis- 
turbed by  any  violent  earth  movements  ;  the  land  was 
then  quietly  worn  down  and  had  an  old  topography,  and 
the  rivers  must  have  carried  comparatively  little  material 
to  the  sea ;  the  sea  would  have  been  bordered  by  low 
coast-lands,  and  cliffs  would  have  been  very  exceptional ; 
and  the  land  would  have  been  protected  from  the  surf  by 
beaches  of  loose  material,  and  some  the  sea  would  in  one 
storm  have  tossed  away  only  to  hurl  it  back  in  the  next. 

In  the  periods  when  the  earth's  crust  was  in  a  quiescent 
stage,  the  wearing  away  of  the  land  and  the  deposition 
of  new  rocks  would  have  been  very  much  slower  than  at 
present,  and  estimates  based  on  the  present  rate  would 
omit  all  those  long  periods  of  the  earth's  history  when  the 
topography  was  mature.  The  earth  is,  in  fact,  recovering 
from  one  of  its  times  of  great  crustal  storm.  The  present 
rates  of  geological  processes  are  therefore  inapplicable  to 
the  long  periods  of  crustal  repose. 

The  age  of  the  earth,  as  determined  by  the  rate  of  sedi- 
mentation, is  doubtless  somewhere  between  the  3  million 
years  estimate  of  Winchell  and  the  6,000  million  years 
thought  necessary  by  McGee. 

The  first  modern  attempt  to  determine  the  age  of  the 
earth  was  by  the  saltness  of  the  sea.  This  method  was 
suggested  in  1715  by  the  astronomer  Halley,  whose  name 
is  well  known,  as  it  has  been  given  to  the  most  famous  of 
comets.  In  later  times  this  method  has  been  used  most 
carefully  by  Prof.  Joly  of  Dublin  and  Dr.  G.  F.  Becker  of 
the  Geological  Survey  of  the  United  States. 

River  water  contains  many  materials  dissolved  in  it ; 
among  the  most  important  of  these  materials  are  car- 
bonate of  lime,  sulphate  of  lime,  common  salt  (chloride  of 

174 


The  Age  of  the  Earth 

sodium),  silica,  chloride  of  potash,  and  various  salts  of  iron 
and  magnesium.  The  Thames,  for  example,  carries  to  the 
sea  every  year  about  550,000  tons  of  mineral  matter  dis- 
solved in  its  waters.  Two-thirds  of  this  amount  are  car- 
bonate of  lime  ;  most  of  the  rest  is  sulphate  of  lime.  The 
salt  amounts  to  about  42,000  tons.  These  materials 
are  continually  poured  into  the  sea,  and  since  evaporation 
removes  only  the  fresh  water,  the  salts  would  accumulate 
unless  they  were  constantly  removed.  They  are  extracted 
from  the  sea  in  different  ways.  Many  animals  and  plants 
have  the  power  of  removing  lime,  with  which  they  form 
their  shells  and  skeletons;  others  extract  the  silica  for  the 
same  purpose.  Iron  and  potash  are  removed  by  the  forma- 
tion of  various  minerals,  such  as  glauconite,  which  accumu- 
late on  the  sea-floor.  The  common  salt,  however,  tends  to 
accumulate  in  the  sea,  which  becomes  steadily  salter.  It 
is  estimated  that  there  are  over  12,000  million  million  tons 
of  sodium  in  the  sea,  and  156  million  tons  are  added  to 
it  every  year ;  there  are  also  454  million  million  tons  of 
potassium,  and  37  million  tons  of  it  are  added  annually. 

If  the  sea  had  been  originally  composed  of  fresh  water, 
and  the  rivers  had  always  carried  into  it  as  much  salt  as 
they  do  now,  then  the  age  of  the  oceans  could  be  deter- 
mined by  dividing  12,000  millions  by  156 ;  and  the  answer 
would  be  that  the  oceans  were  not  quite  77  million  years 
old.  But  if  we  test  the  age  by  the  potassium,  the  calcula- 
tion would  assign  the  world  the  age  of  a  little  over 
12  million  years.  It  is  true  that  some  of  the  potassium 
that  is  being  carried  to  the  sea  by  rivers  is  used  up  in  the 
formation  of  such  minerals  as  glauconite ;  but  the  amount 
of  potassium  taken  from  the  sea-water  in  that  way  would 
hardly  be  sufficient  to  explain  the  great  difference  between 
the  12  million  and  the  77  million  years. 

It  is  therefore  clear  that  the  problem  is  not  so  simple 
as  it  appears  at  first.  The  difficulties  are  manifold.  For 

175 


The  Age  of  the  Earth 

instance,  it  is  improbable  that  the  sea  was  originally 
composed  of  fresh  water,  so  that  time  must  be  deducted 
from  the  answer  to  allow  for  the  amount  of  salt  originally 
present ;  and  as  both  the  size  and  saltness  of  the  primeval 
sea  are  unknown,  this  correction  is  incalculable. 

The  assumption,  moreover,  that  the  amount  of  salt 
added  annually  by  the  rivers  to  the  sea  has  always  been 
the  same  is  very  improbable ;  at  different  times  the  pro- 
portion of  land  to  water  on  the  earth  appears  to  have  been 
different  from  that  at  the  present  time;  and  accordingly 
the  total  amount  of  rainfall  on  the  land  and  the  total 
volume  of  rivers  would  have  been  different. 

Again,  many  rivers  flow  into  basins  where  all  their 
waters  are  lost  by  evaporation,  and  none  ever  reaches  the 
sea ;  and  in  times  when  the  basins  of  inland  drainage  were 
larger  than  at  present  the  quantity  of  water  that  reached 
the  sea  would  have  been  smaller  than  it  is  now,  and  the 
amount  of  the  salt  should  also  have  been  reduced. 

Not  only  would  the  amount  of  water  discharged  to  the 
sea  have  varied,  but  the  proportion  of  salt  in  this  water 
must  have  varied  greatly ;  for,  as  Dr.  Becker  has  remarked, 
in  earlier  times  there  must  have  been  far  larger  areas 
of  igneous  rocks  exposed  on  the  surface  ;  and  as  it  is  from 
these  rocks  that  the  salt  is  originally  derived,  the  waters 
of  the  earlier  rivers  should  have  been  richer  in  salt  than 
those  of  the  present  time. 

Another  series  of  complications  is  due  to  the  fact  that 
salt  is  being  continually  carried  back  from  the  sea  to  the 
land.  Salt  spray  is  raised  from  the  sea  during  storms  and 
blown  ashore,  and  in  seaside  towns  it  seriously  hastens  the 
decay  of  building-stones.  The  spray  is  also  evaporated, 
and  the  tiny  salt-flakes  are  carried  by  the  wind  far  inland. 
This  fact  is  so  well  known  that  maps  are  constructed 
showing  by  lines  known  as  "  isochlors,"  the  varying  con- 
tributions of  sea-salt  to  the  land,  at  different  distances 


The  Age  of  the  Earth 

from  the  shore.  These  isochloric  lines  have  been  deter- 
mined with  especial  care  along  the  coast  of  the  United 
States  between  New  York  and  Boston,  a  district  in  which 
the  amount  of  salt  carried  inland  is  probably  far  below  the 
average.  It  must  be  higher  on  tropical  coasts4  where 
the  prevalent  winds  during  the  day  blow  on  shore,  evapora- 


FIG.  26.— MAP  OF  SOUTHERN  SCOTLAND,  SHOWING  DISTRIBUTION  OF 
SALT  IN  SPRING  AND  SOIL  WATERS.    (AFTER  WILLIAM  BARR.) 

1  =  3f  parts  of  salt  in  100,000  parts  of  water. 

2  —  5  >>  >»  j) 
3=6i              „                   „                  ,, 

4 =  7a  »  »  tt 

tion  is  high,  and  the  sea-water  is  especially  salt.  The 
distribution  of  wind-blown  sea-salt  in  southern  Scotland 
is  shown  in  a  recent  map  by  Dr.  Barr6  (Fig.  26). 

Still  more  important  is  the  incalculable  amount  of  salt 
which  must  have  been  imprisoned  in  the  deposits  laid 
down  on  the  sea-floor.  At  the  time  of  their  deposition 

177  M 


The  Age  of  the  Earth 

these  rocks  must  have  been  saturated  with  salt-water,  and 
much  of  the  salt  (sodium  chloride)  has  probably  been  con- 
verted into  sodium  carbonate,  which  in  many  places  is 
a  more  abundant  constituent  of  sedimentary  rocks  than 
sodium  chloride.  Vast  deposits  of  salt  have  been  also 
removed  from  the  sea  by  the  evaporation  of  lagoons  and 
inland  seas  which  have  been  cut  off  from  the  main  ocean. 

How  much  time  should  be  deducted  from  the  estimate 
to  allow  for  the  salt  already  in  the  primeval  sea  we  do  not 
know.  According  to  the  ingenious  hypothesis  of  Quinton 
and  Macallum,  the  fluid  part  of  the  blood  of  animals  has 
retained  the  chemical  composition  of  the  sea-water,  in 
which  were  living  the  animals  that  first  acquired  closed 
bloodvessels ;  and  according  to  this  view  the  sea  at  an 
early  stage  in  the  development  of  animal  life  had  from 
seven  to  eight  parts  per  thousand  of  mineral  water  instead 
of  thirty-five  parts  per  thousand,  the  proportion  in  the 
present  sea;  and  as  this  stage  must  have  been  passed 
through  long  before  the  beginning  of  Cambrian  times,  the 
sea  probably  had  then  considerably  more  than  eight  parts 
per  thousand  of  salt.  It  is,  however,  possible,  and  even 
probable,  that  the  primeval  sea  was  salter  than  it  is  now, 
and  that  it  has  been  diluted  all  through  geological  time  by 
saltless  water,  which  has  arisen  through  deep  springs  from 
the  interior  of  the  earth. 

In  the  attempt  to  determine  the  age  of  the  earth  by  the 
saltness  of  the  sea,  corrections  have  been  applied  to  the 
results  in  order  to  allow  for  the  numerous  uncertainties, 
Prof.  Joly  has  considered  most  of  the  uncertain  factors, 
and  made  allowances  which  he  regards  as  ample  for  them ; 
thus  for  the  amount  of  salt  returned  from  the  sea  to  the 
land  through  the  atmosphere,  Joly  adds  10  per  cent.,  and 
Becker  6  per  cent.  But  the  uncertainties  are  so  numerous, 
and  each  of  them  has  probably  varied  so  greatly  at  different 
times,  that  the  method  is  ingenious  but  unreliable.  Prof, 

178 


The  Age  of  the  Earth 

Joly  concluded  that  the  age  of  the  oceans  was  from  80 
to  100  million  years ;  Dr.  G.  F.  Becker  accepted  the 
age  as  probably  about  70  millions ;  and  A.  R.  Holmes 
(1913),  after  a  careful  study  of  the  whole  question,  con- 
cluded that  the  limits  given  by  this  method  would  be 
between  210  and  340  million  years ;  but  he  remarks  that 
the  figures  "  must  not  be  supposed  to  possess  any  serious 
value.  The  whole  discussion  merely  serves  to  betray  the 
uncertainty  of  the  method,  and  the  doubtful  applicability 
of  even  the  most  accurate  data  "  (p.  74). 

The  simple  division,  therefore,  of  the  amount  of  salt  in 
the  sea  by  the  estimated  annual  contribution  of  salt  from 
the  land  to  the  sea  does  not  give,  with  any  reliability,  the 
age  of  the  oceans.  It  is  a  complex  equation,  with  many 
factors,  nearly  all  of  which  are  unknown.  How  salt  the 
sea  was  to  start  with ;  what  have  been  the  variations  in 
the  annual  contributions  of  salt  from  the  land  to  the  sea ; 
how  much  salt  has  been  returned  from  the  sea  to  the  land — 
all  give  uncertain  replies.  The  conclusions  based  on  this 
method  are  very  interesting,  but  too  uncertain  to  be  of 
serious  weight  if  they  conflict  with  the  other  geological 
evidence  and  requirements. 

The  claim  by  geologists  that  the  age  of  the  world  must 
be  counted  in  hundreds  of  millions  of  years  has  been 
resisted  by  the  physicists  on  the  ground  that  the  heat 
supply  of  the  Solar  System  is  limited.  The  most  famous 
of  physical  attempts  to  discover  the  maximum  age  of  the 
earth  was  that  made  by  Lord  Kelvin,  when  he  proposed 
to  restrict  its  age  to  a  comparatively  short  period.  A 
furnace  will  not  continue  to  burn  unless  it  be  supplied 
with  fresh  fuel ;  the  sun  cannot  continue  to  give  off  heat 
indefinitely.  According  to  Lord  Kelvin,  unless  the  sun 
has  been  recharged  with  fuel  it  cannot  have  begun  to  burn 
more  than  500  million  years  ago,  and  probably  not  more 
than  100  million  years  ago;  and  as  the  history  of  the 

179 


The  Age  of  the  Earth 

earth  began  long  after  the  beginning  of  the  sun,  geological 
time  must  be  much  less  than  500  million  years  and 
probably  much  less  than  100  million.  Lord  Kelvin 
subsequently  strengthened  this  conclusion  by  arguments 
from  the  internal  temperatures  of  the  earth.  It  is  well 
known  that  the  temperature  of  the  earth  increases  below 
the  surface.  This  fact  has  been  proved  by  observations  in 
deep  mines,  where  the  intense  heat  is  one  of  the  greatest 
obstacles  to  profitable  mining.  Lord  Kelvin  calculated 
from  this  rate  of  increase  in  underground  temperature  that 
the  crust  of  the  earth  must  be  at  least  20  million  and  not 
more  than  400  million  years  old,  and  that  it  was  probably 
formed  about  100  million  years  ago.  Subsequently,  in 
1876,  he  reduced  these  limits  and  gave  from  50  to 
go  million  years;  and  later,  in  1897,  he  again  lowered 
the  times  and  gave  from  20  to  40  million  years.  And 
Prof.  Tait,  in  1875,  was  even  more  niggardly  in  the  time 
he  would  admit  as  possible ;  he  would  only  allow  10  million 
years  since  the  earth  first  became  cool  enough  on  the 
surface  for  the  lowest  forms  of  vegetable  life  to  have  existed. 

Ten  million  years  was  obviously  quite  inadequate,  and 
though  some  geologists  felt  that  the  physical  arguments 
looked  so  precise  that  they  must  be  accepted,  others 
rejected  them  as  absolutely  contradicted  by  the  geological 
facts.  Huxley,  with  characteristic  insight,  remarked  that 
mathematics  was  a  mill  which  ground  up  material,  but 
that  the  nature  of  the  results  entirely  depended  on  the 
material  put  into  it ;  and  if  the  data  are  uncertain,  the 
results  must  be  unreliable.  One  critic  insisted  that  Lord 
Kelvin  and  Prof.  Tait  had  omitted  various  factors  which 
would  further  reduce  their  estimates,  with  the  result  that 
the  time  from  the  Cambrian  period  to  the  present  day 
amounted  to  3  million  years  less  than  nothing. 

The  short  estimates  of  geological  time  which  some 
geologists  have  been  ready  to  accept  were  doubtless  due 

180 


The  Age  of  the  Earth 

to  the  influence  of  the  physical  arguments.  Since  1895, 
however,  various  physicists  have  come  to  the  aid  of  geology 
and  shown  that  the  scepticism  of  Huxley  was  fully  justified. 

In  1894,  Lord  Salisbury,  in  his  Presidential  Address  to 
the  British  Association  at  Oxford,  accepted  Lord  Kelvin's 
limit  to  the  age  of  the  earth  and  then  claimed  that  the 
doctrine  of  evolution  must  be  dismissed  as  there  was  no 
adequate  time  for  it.  Lord  Salisbury  thus  did  geology  a 
service  of  the  highest  value.  When  faced  by  this  dilemma 
Prof.  Perry,  who  had  previously  accepted  Lord  Kelvin's 
conclusions  as  sound,  was  led  to  re-examine  them ;  and 
he  then  recognized  that  by  modifying  Lord  Kelvin's 
assumptions  as  to  the  internal  structure  of  the  earth,  the 
age  which  Lord  Kelvin  was  prepared  to  grant  might 
be  multiplied  very  many  times.  Prof.  Tait,  who  had 
characteristically  cut  down  Lord  Kelvin's  minimum  by 
half,  and  had  limited  the  earth  to  an  age  of  10  million 
years,  promptly  took  up  the  cudgels  on  Lord  Kelvin's 
behalf,  and  he  held  that  it  was  for  Prof.  Perry  to  prove 
that  the  interior  of  the  earth  was  not  what  Lord  Kelvin 
had  assumed.  This  was  an  entirely  unjust  claim.  It  was 
for  Lord  Kelvin  and  those  who  accepted  his  limit  to  prove 
the  assumption  on  which  it  was  based ;  and,  as  a  matter 
of  fact,  Prof.  Perry's  assumption  is  by  far  the  more 
probable.  Lord  Kelvin  had  assumed  that  heat  would  be 
conducted  through  the  materials  in  the  interior  of  the 
earth  at  about  the  same  rate  as  through  the  rocks  of 
the  crust ;  whereas  the  overwhelming  balance  of  the 
evidence  now  available  is  in  favour  of  the  view  that 
the  materials  in  the  centre  of  the  earth  would  conduct 
heat  far  more  readily  than  the  rocks  of  the  crust. 

According  to  Prof.  Perry,  if  heat  passes  through  the 
materials  four  miles  deep  ten  times  as  quickly  as  through 
the  rocks  near  the  surface,  then  the  present  rate  of  increase 
in  underground  temperature  can  be  explained  even  if  the 

181 


The  Age  of  the  Earth 

age  of  the  earth  were  10,000  million  years;  and  Lord 
Kelvin  himself  admitted  that  he  should  perhaps  have 
allowed  4,000  million  years  as  the  possible  age  of  the 
earth  as  based  upon  that  argument. 

But  since  that  date  the  arguments  both  from  the  rate 
of  increase  of  underground  temperature  and  from  the  heat- 
supply  of  the  sun  have  been  absolutely  put  out  of  court  by 
the  discovery  of  radium,  for  the  heat  of  the  crust  may  be 
maintained  by  the  action  of  radium ;  and  Prof.  Perry  has 
pointed  out  that  even  if  Prof.  Strutt  had  overestimated 
the  amount  of  radium  in  the  earth's  crust  by  twenty  times, 
we  might  still  multiply  the  age  which  Lord  Kelvin  admitted 
for  the  earth  by  one  thousand.  Any  geologist  who  is  not 
content  with  that  amount  of  time  is  greedy. 

"We  are  now,"  says  Prof.  Perry,  "  in  a  position  to  say 
that  the  physicist  can  make  no  calculation  either  as  to  the 
probable  or  possible  age  of  life  on  the  earth."6 

The  conclusion  that  the  geological  and  biological 
evidence  as  to  the  age  of  the  earth  is  more  reliable  than 
the  physical  was  independently  reached  a  few  years  later 
by  Dr.  Nils  Ekholm.  He  modified  one  of  Lord  Kelvin's 
assumptions,  and  thus  obtained  an  age  for  the  earth  of 
"  many  thousand  million  years."7  He  claims  to  have 
shown  that  "  no  valid  reasons  against  the  estimate  of  the 
age  of  life  on  the  earth  made  by  geologists  and  biologists 
can  be  taken  from  the  laws  and  facts  obtained  from 
physical  researches.  It  also  seems  to  me  that  the 
geological  and  biological  facts  on  which  this  estimate 
is  founded  are  at  least  as  reliable  as  the  physical  constants 
and  assumptions  on  which  a  calculation  of  the  secular 
cooling  of  the  earth  or  the  heat-store  of  the  sun  are 
based.  Still  much  more  unreliable  are  the  calculations 
of  the  earth's  age  founded  on  a  hypothesis  as  to  the 
mode  of  formation  of  the  moon  or  the  tidal  friction 
exercised  by  it  on  the  earth.  For  so  long  as  Laplace's 

182 


The  Age  of  the  Earth 

nebular  hypothesis  has  not  been  verified  by  an  exact 
mathematical  analysis  in  all  its  consequences  and  details, 
we  know  nothing  about  the  age  of  the  moon  as  a  satellite 
of  our  earth." 

The  argument  from  underground  temperatures  has 
therefore  been  dismissed,  and  the  case  for  the  cooling  of 
the  sun  has  also  been  overthrown.  Lord  Kelvin  admitted 
that  his  time  allowances  were  only  valid  provided  that 
there  were  no  available  unknown  source  of  heat.  And 
such  a  source  of  heat  was  revealed  by  the  discovery  of 
radium.  For  radium  has  the  property  of  breaking  up  into 
other  bodies  and  giving  out  heat  during  the  process. 
Radium  is  widely  distributed  in  the  earth,  and  there  is 
so  much  of  it  and  it  gives  forth  so  much  heat  that  the 
wonder  is  that  the  earth  is  not  much  hotter  than  it  is. 
The  sun  also  contains  radio-active  bodies,  and  though 
they  are  inadequate  to  cause  any  appreciable  part  of  its 
heat,  yet  it  is  held  by  some  authorities  that  the  heat  of 
the  sun  may  be  maintained,  by  structural  changes  in  its 
materials,  for  sufficient  periods  of  time  to  satisfy  all  the 
requirements  of  geologists. 

Radium,  however,  in  addition  to  its  indirect  evidence 
as  a  formerly  unsuspected  source  of  heat,  has  given  un- 
expected direct  testimony  to  the  antiquity  of  the  earth. 
The  metal  uranium  breaks  up  into  different  materials  of 
which  one  is  the  element  known  as  helium,  and  this  in 
turn  is  believed  to  be  converted  after  a  long  series  of 
changes  into  lead.  If,  then,  a  uranium  mineral  is  being 
slowly  converted  to  lead,  an  old  uranium  mineral  should 
contain  more  lead  than  a  specimen  of  the  same  mineral  of 
more  recent  formation ;  and  as  helium  is  also  formed  from 
the  uranium  and  accumulates  in  the  mineral,  the  amount 
of  helium  is  also  a  test  of  the  age  of  the  mineral. 

Either  lead  or  helium  may  then  be  used  as  a  test  of  the 
age  of  a  uranium  mineral. 

183 


The  Age  of  the  Earth 

As  soon  as  the  possibility  of  using  this  method  was 
recognized,  Prof.  Strutt  made  many  experiments  to  deter- 
mine the  age  in  years  of  the  fossiliferous  rocks ;  and  he 
determined  the  amount  of  helium  in  the  fragments  of 
phosphate  and  fossil  bones  in  many  different  rocks.  Some 
of  his  results  are  as  follows  : 


Material. 

Geological  Horizon. 

Locality. 

Years. 

Phosphatized  whale- 

Red Crag  (Pliocene) 

Felixstowe 

II2,OOO 

bones 

Phosphatic   nodules 

Red  Crag  (Pliocene) 

Felixstowe 

225,000 

Phosphatic   nodules 
Phosphatic   nodules 

Upper  Greensand 
Lower  Greensand 

Cambridge 
Potton,    Bed- 

3,080,000 
3,950,000 

fordshire 

Phosphatized  reptile 

Kimmeridge  Clay 

Ely 

Over 

bones 

1,210,000 

Phosphatized  reptile 

Oxford  Clay 

Whittlesea 

Over 

bones 

6,110,000 

Phosphatic    bone 

Rhaetic    Bone    Bed 

Lyme  Regis 

Over 

fragments 

(Base  of  Jurassic) 

1,120,000 

Haematite    

Above  Carboniferous 

Frizington, 

141,000,000 

Limestone 

Cumberland 

Phosphatic   nodules 

Bala  Beds 

Near  Bala 

51,900,000 

Phosphatic  lime- 

Llandeilo Limestone 

Cherbury, 

77,900,000 

stone 

Shropshire 

Phosphatic   nodules 

Torridon  Sandstone 

Cailleach 

9,250,000 

Head,  Loch 

Broom 

The  dates  in  this  list  are  inconsistent.  The  Torridon 
Sandstone  is  far  older  than  the  Carboniferous  Lime- 
stone instead  of  being  much  more  recent,  and  the  paltry 
age  of  100,000  years  assigned  to  whale-bones  from  the 
Red  Crag  of  Felixstowe  is  quite  inadequate.  The  dif- 
ferences are  attributed  to  some  of  the  helium  generated 
having  leaked  out  of  the  phosphate,  so  that  the  age  it  gives 
may  be  too  small.  Prof.  Strutt  therefore  tested  minerals 
such  as  iron  ores  and  zircons,  which  would  retain  their 
helium  better,  and  these  indicate  ages  of  even  greater 
antiquity.  Thus  the  age  of  the  zircons  of  Monte  Somma, 
the  old  crater  of  Vesuvius,  is  100,000  years;  of  the  Miocene 

184 


The  Age  of  the  Earth 

volcanoes  of  Central  France  6,300,000  years ;  and  the 
zircons  from  the  Devonian  rocks  of  Brevig  in  Norway 
as  54  million  years. 

The  conclusions  based  on  the  amount  of  the  lead  present 
in  the  uranium  minerals  assign  to  the  Carboniferous  the 
date  of  340  million  years  ago,  the  Devonian  370  million, 
the  Silurian  430  million,  and  the  Archean  as  1,000  to 
i, 600  million  years  ago. 

The  method  is  attended  with  various  uncertainties ; 
thus,  some  of  the  lead  may  have  been  present  originally 
in  the  mineral,  and  Becker  has  shown  that  a  series  of 
uranium  minerals,  all  from  the  same  locality  in  Texas,  and 
of  the  same  age,  gave  very  different  results,  the  lead  ratio 
in  some  being  seven  times  as  great  as  in  others.  But  this 
has  been  explained  as  due  to  alterations  in  the  minerals 
since  their  formation,  so  that  some  of  the  lead  formed 
may  have  been  removed  during  these  changes.  Moreover, 
it  is  possible,  as  suggested  by  Joly,  that  the  uranium  may 
have  broken  down  more  quickly  in  the  early  stages  than 
in  the  later,  so  that  the  age  indicated  by  the  lead  forma- 
tion may  be  greatly  exaggerated. 

Boltwood  assigns  an  age  from  246  to  1,320  million  of 
years  to  some  uranium-bearing  minerals ;  but  Prof.  Joly 
holds  that  the  evidence  from  the  rate  of  rock  formation 
and  from  the  amount  of  salt  in  the  sea  shows  that  the 
radio-active  estimates  cannot  be  accepted.  But  his  own 
re-discussion  of  the  argument  from  the  saltness  of  the  sea 
only  serves  to  indicate  the  extreme  uncertainty  of  this 
argument,  for  the  source  of  the  chlorine  in  the  salt  has 
not  itself  received  any  satisfactory  explanation.  The  only 
considerable  source  of  chlorine  is  that  which  is  discharged 
from  volcanoes  and  hot  springs,  and  at  any  period  of 
volcanic  quiescence  the  amount  of  chlorine  thus  dis- 
charged to  the  atmosphere  would  be  much  smaller,  and 
thus  the  sodium  would  be  carried  to  the  sea  in  some  other 

185 


The  Age  of  the  Earth 

form  than  sodium  chloride.  The  fact  that  we  are  now 
in  a  period  of  more  than  usual  volcanic  activity  again 
indicates  that  the  amount  of  salt  (sodium  chloride)  being 
carried  to  the  sea  is  above  the  average. 

The  method  of  measuring  geological  time  by  the  radio- 
active products  offers  most  promise  of  a  definite  result. 
Geologists  readily  welcome  results  which  assign  to  the 
Archean  period  an  antiquity  of  from  1,000  to  2,000  million 
years ;  but  it  must  be  admitted  that  the  results  are  not 
sufficiently  consistent  to  be  reliable  as  absolute  figures. 
But  the  radio-active  method,  the  most  hopeful  of  all 
physical  evidence  as  to  the  age  of  the  earth,  agrees  with 
the  geological  evidence  that  the  age  of  the  earth  must  be 
reckoned  in  periods  of  time  so  great  that  geologists  may 
assume  that  the  Archean,  the  first  geological  era,  came  to 
an  end  1,000  million  years  ago. 

Other  geologists,  however,  felt  convinced  by  the 
physical  arguments,  and  were  prepared  to  confine  the 
age  of  the  earth  within  the  time  estimated  by  Kelvin  and 
Tait,  even  if  that  involved  compressing  the  deposition  of 
all  the  known  fossiliferous  rocks  within  some  such  period 
as  3  million  years.  If  it  were  absolutely  proved  that  the 
earth  is  only  a  few  million  years  old,  geologists  would 
have  to  assume  that  we  are  living  in  a  period  of  excep- 
tional quiescence,  and  geological  history  would  have  to 
be  rewritten  with  all  geological  operations  greatly 
quickened.  It  would  have  to  be  assumed  that  the  earth's 
crust  is  so  mobile  that  the  lands  have  nearly  always 
been  kept  at  a  high  level,  and  with  steep  slopes  to  the 
sea,  so  that  the  rivers  were  always  swift ;  and  hence  both 
the  wearing  down  of  the  land  and  the  deposition  of  fresh 
rocks  were  far  more  rapid  than  at  present.  The  folding 
of  the  crust  into  mountain-chains  must  also  have  occurred 
at  short  intervals,  and  the  violent  earth  movements  would 
have  led  to  volcanic  eruptions  of  great  frequency  and 

186 


The  Age  of  the  Earth 

intensity.      The  volcanoes   would    have    poured  carbon 
dioxide  into  the  atmosphere  far  more  rapidly  than  it  could 
have  been  absorbed,  and  these  changes  in  the  composition 
of  the  atmosphere  would  have  occasioned  rapid  climatic 
variations  which  would  have  exterminated  many  organisms, 
and  have  led  to  the  rapid  evolution  of  others  to  take  their 
place.     The   assumption   that   the  earth  has  been   con- 
tracting so  rapidly  that  its  surface  has  been  constantly 
disturbed   by  severe  crustal  storms  would  involve  such 
a  speeding-up  of  terrestrial  evolution  that  its  whole  history 
might  have  been  completed  within  a  few  million  years. 
The  evidence,  however,  appears  so  overwhelming  against 
any  such  series  of  assumptions  that  geologists  are  not 
likely  to  accept  it  without  convincing  evidence;  and  in 
this  attitude  they  are  confirmed  by  zoologists,  who  con- 
sider that  the  development  of  animal  life  on  the  earth 
must  have  required  a  period  which  can  only  be  measured 
in  hundreds  of  millions  of  years.     Thus,  Prof.  Poulton, 
in  his  address  to  the  British  Association  in  1896,  insisted 
that   even   if    geologists   were    prepared    to    accept   the 
physicists'  restrictions  on  the  antiquity  of  the  earth,  as 
some  were  then  prepared  to  do,  the  evidence  of  zoology 
would  alone  be   sufficient  to   refute   them.     Subsequent 
work  upon  the  rate  of  evolution  and  on  the  age  of  the 
earth  has  justified  Prof.  Poulton's  conclusion. 

How  old  the  world  is  cannot  be  definitely  fixed,  but  the 

period   covered   by   geological   evidence   must   be   many 

hundreds,  and  may  be  many  thousands,  of  millions  of  years.8 

The  poet  Cowper,  in  an  unsympathetic  account  of  the 

proceedings  of  geologists,  said  : 

"  Some  drill  and  bore 
The  solid  earth,  and  from  the  strata  there 
Extract  a  register,  by  which  we  learn 
That  He  who  made  it,  and  reveal'd  its  date 
To  Moses,  was  mistaken  in  its  age." 

187 


The  Age  of  the  Earth 

If  Cowper  were  now  to  prepare  a  new  edition  of  that 
poem  he  might  claim  that  those  who  drill  and  bore  the 
earth  have  also  proved  the  error  of  many  more  recent 
estimates,  which  have  assigned  a  limited  antiquity  to  this 
venerable  earth. 

1  A.  Winchell,   "  World  -  Life  :   A  Comparative  Geology,"  third 
edition  (Chicago,  1889),  p.  378. 

2  W.  J.  McGee,  "  Note  on  the  Age  of  the  Earth,"  Science,  1893, 
vol.  xxi.,  pp.  309,  310. 

3  The  area  of  the  United  States  is  3,088,500  square  miles,  and  that 
of  the  Mississippi  Basin   1,265,000  square  miles.     The  amount  of 
sediment  carried  to  the  sea  by  the  Mississippi  is  304  million  tons, 
and  that  by  all  the  remaining  rivers  of  the  United  States  is  only 
164  million  tons. 

4  Sir  Thomas  Holland  has  recently  calculated  from  observations 
in  India  that  the  amount  of  saltthus  carried  inland  does  not  seriously 
affect  the  question. 

5  The  clearest  general  summary  of  the  arguments  on  this  question 
is  given  by   Prof.   Poulton  in  his  "  Essays  on    Evolution,"   1908, 
pp.  1-16. 

c  E.  B.  Poulton,  "  Essays  on  Evolution,"  1908,  p.  15. 

7  N.  Ekholm,  Quart.  Journ.  Roy.  Meteor.  Soc.,  1901,  vol.  xxvii., 
pp.  n,  14,  etc. 

8  For  a  recent  summary  of  the  work  on  the  age  of  the  earth,  see 
Mr.  Holmes's  book,  "  The  Age  of  the  Earth,"  and  a  series  of  articles 
by  H.  S.  Shelton,  of  which  the  last  includes  reference  to  his  former 
papers  ("  Some  Aspects  of  Geologic  Time,"  Science  Progress,  October, 
1913,  vol.  v.,  pp.  250-274). 


188 


CHAPTER  XII 
THE  ERA  OF  THE  DAWN  OF  LIFE 

i.  THE  USE  OF  FOSSILS. 

HISTORICAL  geology  deals  with  the  history  of  the  earth  as 
recorded  by  the  successive  rocks  which  make  up  its  outer 
crust.  The  interpretation  of  the  evidence  of  the  rocks 
requires  the  determination  of  their  ages  in  different  parts 
of  the  world,  just  as  in  writing  the  history  of  man  it  is 
necessary  to  determine  the  dates  of  different  civilizations 
and  nations. 

The  age  of  a  rock  is  determined  by  the  fossils  contained 
in  it,  and  the  discovery  how  to  use  fossils  earned  for 
William  Smith,  the  Bath  land  surveyor,  his  title  of  "  the 
Father  of  Geology."  He  recognized  that  rocks  laid  down 
at  the  same  period  contained  the  same  kind  of  fossils  ; 
fossils,  therefore,  might  be  used  to  date  the  rocks  in 
which  they  occur,  just  as  ruins  of  ancient  cities  are  dated 
by  the  coins  buried  in  them.  Fossils,  in  fact,  have  been 
called  "  the  medals  of  creation,"  because  they  help  the 
geologist  in  the  same  way  that  coins  and  medals  help  the 
antiquarian  and  historian. 

According  to  the  nature  of  their  fossils  the  stratified 
rocks  are  divided  into  four  main  Groups,  and  the  Groups 
are  subdivided  into  Systems,  and  these  into  Series.  Each 
Group  of  rocks  was  laid  down  in  a  separate  subdivision  of 
geological  time  known  as  an  Era. 

189 


The  Era  of  the  Dawn  of  Life 

The  subdivision  of  these  Groups  and  their  time 
equivalents  are  shown  on  the  following  table : 

Time- 
equivalent. 
Group      ...  ...  ...  ...     Era. 

System    ...  ...  ...  ...     Period. 

Series      ...  ...  ...  ...     Epoch. 

Stage       Age. 

When  these  terms  are  used  in  this  book  with  these 
narrowly  defined  meanings,  they  begin  with  a  capital 
letter. 

2.  THE  Eozoic  ERA. 

The  first  of  these  four  Groups  is  the  Eozoic,  so  called 
because  it  was  the  time  of  the  dawn  of  life  on  the  earth. 
Animals  in  their  larval  or  embryonic  stages  are  soft- 
bodied.  They  have  no  hard  skeletons  or  shells,  and  it  is 
probable  that  the  first  animals  which  lived  upon  the  earth 
were  also  devoid  of  hard  structures.  As  a  rule  it  is  only 
the  hard  parts  of  animals  which  are  found  as  fossils. 
Occasionally  soft-bodied  animals  like  jelly-fish  leave 
impressions  on  mud  which  may  be  preserved  as  fossils. 
But  such  impressions  are  only  retained  in  rocks  which 
have  undergone  very  slight  subsequent  changes.  Hence 
it  is  not  surprising  that  practically  no  fossils  have  yet  been 
discovered  in  the  whole  of  the  vast  thickness  of  Eozoic 
rocks ;  for  the  animals  of  that  Era  were  soft-bodied,  and 
the  rocks  have  in  most  cases  been  so  intensely  altered 
that  only  the  remains  of  animals  with  exceptionally 
massive  skeletons  or  shells  would  have  any  chance  of 
preservation.  The  Eozoic  group  consists  mainly  of 
igneous  rocks  and  of  others  such  as  gneisses  and  schists, 
which  have  been  formed  by  alteration  due  to  heat  and 
pressure ;  and  in  these  changes  the  original  structures 
have  been  obliterated. 

The  Eozoic  rocks  include,  however,  two  main  types. 

190 


The  Era  of  the   Dawn   of  Life 

The  older  rocks  of  this  Group  have  been  so  altered  that 
most  of  them  have  been  crystallized.  Their  arrangement 
in  the  field,  however,  shows  that  many  of  them  were  laid 
down  as  widespread  sheets  of  sediments.  Many  of  them 
were  no  doubt  originally  beds  of  clay  and  sand.  The 
beds  have  still  the  arrangement  of  a  stratified  series, 
though  they  have  been  altered  into  metamorphic  rocks. 

Occasionally  structures  have  been  found  in  these  Eozoic 
rocks  which  have  been  described  as  fossils.  The  most 
famous  of  them  is  the  Eozoon  (dawn-animal),  which  was 
found  in  eastern  Canada,  and  explained  as  a  gigantic  reef- 
building  animal  of  a  very  primitive  kind.  It  was,  how- 
ever, formed  by  the  alteration  of  blocks  of  limestone  which 
were  immersed  in  molten  rock  and  absorbed  some  of  the 
constituents,  and  were  thus  converted  into  alternate  layers 
of  crystalline  limestone  and  of  various  igneous  minerals. 
This  interpretation  was  first  worked  out  from  limestone 
blocks  found  in  the  ancient  lavas  of  Vesuvius,  which  have 
a  similar  structure;  but  the  same  interpretation  applies  to 
the  Canadian  specimens  from  the  typical  locality  of  Cote 
St.  Pierre. 

Though  the  Lower  Eozoic  rocks  have  not  yet  yielded 
any  definite  fossils,  there  is  indirect  evidence  of  the 
existence  of  life  during  their  deposition,  for  the  rocks 
include  sheets  of  limestone  which,  though  now  crystalline, 
were  probably  formed  by  the  action  of  animal  life.  Some 
graphite,  which  is  composed  of  carbon,  and  some  bitumen 
also  occur,  and  they  may  be  the  remains  of  plants.  The 
existence  of  plants  at  this  period  necessarily  follows  from 
the  occurrence  of  animals,  which  were  no  doubt  as 
dependent  for  food  on  plants  as  those  of  later  geological 
periods. 

The  Upper  Eozoic  rocks  are  very  different  in  character 
from  those  of  the  lower  part  of  the  Group.  They  are 
composed  of  sandstones  and  conglomerates  and  other 

191 


The  Era  of  the  Dawn  of  Life 

rocks  which  are  so  little  altered  that  they  cannot  be 
distinguished  from  similar  rocks  of  later  geological 
Periods.  All  over  the  world  these  Upper  Eozoic  rocks 
were  originally  assigned  to  later  geological  Eras.  Thus 
the  rocks  of  the  Rand  goldfield  were  regarded  as  the  same 
age  as  the  Table  Mountain  Sandstone  beside  Capetown ; 
and  the  Torridon  Sandstones  of  Scotland  were  described 
by  Hugh  Miller  as  the  Old  Red  Sandstone.  The  Table 
Mountain  and  Old  Red  Sandstones  both  belong  to  the 
period  known  as  the  Devonian,  and  yet  the  Rand  and 
the  Torridon  Sandstones  are  now  regarded  as  Eozoic. 

These  unaltered  Upper  Eozoic  rocks  have  yielded 
obscure  traces  of  life.  The  Torridon  Sandstone  includes 
some  grains  of  phosphates  of  lime,  in  which  Dr.  Hinde 
has  recognized  organic  structures.  Dr.  Walcott  has 
described  fossils  from  rocks  of  about  the  same  age  in 
Montana  in  the  United  States  ;  the  most  important  of 
these  fossils  is  a  Crustacean  known  as  Beltina,  which  is 
the  best  known  of  Eozoic  fossils.  Dr.  Walcott  has 
recently  (1912)  described  some  fossils  from  a  Canadian 
Limestone,  which  is  referred  to  the  Lower  Eozoic ;  and 
if  the  age  of  the  rock  be  correct,  this  is  the  oldest  form  of 
life  yet  known.  The  fossil,  Atikokania  lawsoni,  is  allied  to 
the  sponges.  Dr.  Walcott  describes  it  as  similar  in  aspect 
to  some  from  early  Palaeozoic  Limestones,  and  expresses 
surprise  that  such  a  fossil  should  be  found  in  Lower 
Eozoic  rocks.  Its  age  cannot  therefore  be  regarded  as 
definitely  established. 

Owing  to  the  altered  conditions  of  the  Lower  Eozoic 
rocks  we  have  no  satisfactory  evidence  as  to  the  con- 
ditions under  which  they  were  laid  down,  but  they  were 
probably  largely  deposited  beneath  the  sea.  During  their 
formation  the  earth  was  disturbed  by  widespread  volcanic 
eruptions.  The  amount  of  rocks  which  were  deposited 
during  this  period  as  ordinary  sediments  and  then  altered 

192 


The  Era  of  the  Dawn  of  Life 

into  schists  was  so  colossal,  that  these  Lower  Eozoic  rocks 
form  an  almost  continuous  foundation  for  the  whole  crust 
of  the  earth.  They  are  exposed  over  a  larger  proportion 
of  the  surface  of  the  earth  than  rocks  of  any  other  group. 
They  form  the  foundation  of  Scandinavia  and  Finland,  of 
eastern  Canada,  the  peninsula  of  India,  as  well  as  large 
parts  of  northern  Asia,  and  most  of  Africa  and  of  Australia. 
If  these  Eozoic  sediments  were  laid  down  at  the  same 
rate  as  those  of  later  times,  then  the  length  of  the  Eozoic 
Era  was  perhaps  as  great  as  that  of  the  whole  of  later 
geological  time. 

The  Upper  Eozoic  rocks  are,  however,  so  little  altered 
that   they   indicate   clearly   the   geographical   conditions 
under   which   they   were    formed.      The   most   extensive 
representative  of  this  formation  in  the  British  Isles  is  the 
Torridon  Sandstone  in  north-western  Scotland,  and  the 
nature   of  this   rock   shows   it  was   deposited  when   the 
country  was  a  desert.     The   pebbles   in   the   sandstones 
still  show  the  marks  of  wind  action,  for  the  sand-grains 
blown  against  them  by  the  wind  have  cut  them  into  the 
characteristic  form  of  desert  stones.     The  sandstone  was 
laid  down  on  an  old  land  surface,  the  valleys  of  which  are 
still  preserved  as  they  were  filled  up  with  these  ancient 
sandstones.     The   mountains   of  western   Australia  have 
been  smothered  by  wind-blown  earth,  until  by  the  com- 
bined wearing  down  of  the  ridges  and  filling  up  of  the 
valleys   vast   areas    have   been   converted   into   a   gently 
undulating  plain.     The  Torridon  Sandstones  have  simi- 
larly smothered  the  old  mountains  of  Lower  Eozoic  rocks 
and  filled  in  the  valleys,  which  are  being  again  exposed 
by  the  slow  removal  of  the  sandstone. 

The  existence  of  a  desert  climate  shows  that  the 
arrangement  of  land  and  water  in  the  British  area  was 
very  different  from  that  of  the  present  time.  The  size 
of  the  sand-grains  that  were  then  moved  by  the  wind 

193  N 


The   Era  of  the   Dawn  of  Life 

indicates  that  it  then  blew  with  much  the  same  force  as 
it  does  to-day,  and  the  position  of  the  sand-worn  faces  of 
the  pebbles  shows  that  the  prevalent  winds  came  then,  as 
now,  from  the  south-west.  If  there  had  been  a  great  ocean 
to  the  west  of  the  British  Isles  the  winds  would  have  been 
moist  instead  of  dry ;  so  we  may  safely  conclude  that  the 
North  Atlantic  was  not  then  in  existence,  and  the  west 
winds  were  dried  before  reaching  Scotland  by  having 
passed  over  a  wide  expanse  of  land. 

Traces  of  ice  action  and  of  the  existence  of  glaciers  in 
the  Eozoic  rocks  prove  that  the  earth's  climate  was  not 
then  much  hotter  than  at  present.  Glacial  deposits  of 
Lower  Eozoic  Age  have  been  discovered  by  Prof.  Coleman 
at  Cobalt  in  eastern  Canada,  and  those  of  Upper  Eozoic 
Age  are  known  in  northern  Norway  and  Spitsbergen. 
Hence  glaciers  existed  on  the  earth  in  the  first  period  of 
geological  history.  All  through  geological  time  the  climate 
of  the  earth  appears  to  have  varied  between  the  same 
limits  as  those  which  are  found  on  the  earth  at  present. 
Some  districts  were  waterless,  some  mountains  were  snow- 
capped, and  some  valleys  were  occupied  by  glaciers  at  the 
very  beginning  of  geological  time. 

The  Absence  of  Eozoic  Fossils. — The  second  geological 
Group  is  known  as  the  Palaeozoic,  because  the  earth  was 
then  inhabited  by  animals  of  the  oldest  known  types-;  and 
the  most  startling  change  from  the  Eozoic  conditions  was 
the  appearance  in  the  very  oldest  of  the  Palaeozoic  rocks 
of  abundant  remains  of  highly  specialized  and  very  varied 
kinds  of  animals.  A  complex  assemblage  of  animals 
bursts  upon  the  scene  with  dramatic  suddenness  in  the 
earliest  section  of  the  Palaeozoic  Era.  The  oldest  Palaeo- 
zoic group  of  animals  includes  those  highly  specialized 
crustaceans,  the  trilobites,  shrimp -like  animals,  corals, 
also  lamp-shells,  worms,  sea-lilies,  starfish,  sea-cucumbers, 
and  many  kinds  of  shellfish,  including  groups  of  which 

194 


The  Era  of  the  Dawn  of  Life 

the  modern  representatives  are  the  mussel,  whelk,  and 
nautilus. 

The  geological  record  of  life  on  the  earth  begins,  indeed, 
with  well-developed  representatives  of  all  the  chief  groups 
of  the  animal  kingdom,  with  the  exception  of  the  back- 
boned animals.  There  are  no  known  land  plants  which 
were  contemporary  with  Olenellus,  though  they  were 
doubtless  in  existence,  and  no  remains  of  insects  or  other 
land  animals  of  that  time  have  been  found.  The  sudden 
appearance  of  so  many  groups  of  highly-developed  crea- 
tures at  the  beginning  of  the  Palaeozoic  is  the  most 
striking  fact  in  regard  to  these  oldest  fossil-bearing  rocks. 
The  animals  must  have  had  almost  innumerable  genera- 
tions of  ancestors,  which  lived  in  the  Eozoic  Era ;  even 
if  they  had  had  thick  hard  shells  they  would  not  have 
been  preserved  as  fossils  in  the  Lower  Eozoic  rocks, 
yet  they  should  have  been  found  in  the  unaltered  Upper 
Eozoic  shales.  The  absence  of  fossils  from  the  unaltered 
Eozoic  rocks  is  probably  due  to  the  fact  that  the  animals 
then  had  no  hard  shells  or  skeletons.  We  have,  there- 
fore, sadly  to  realize  that  there  is  no  prospect  of  rinding 
fossil  remains  before  the  beginning  of  Palaeozoic  times. 
No  doubt  discoveries  will  be  made  from  time  to  time  of 
farther  traces  of  Eozoic  life,  but  there  appear  no  grounds 
for  hope  that  we  shall  ever  find  any  representative  col- 
lections of  Eozoic  fossils  such  as  are  found  in  all  subse- 
quent geological  periods. 

Two  explanations  have  been  offered  of  the  absence  of 
shells  before  the  Palaeozoic  Era.  One  which  is  attractive 
from  its  simple  ingenuity  is  based  on  the  view  that 
skeletons  and  shells  are  mainly  developed  for  defence 
against  carnivorous  organisms.  This  explanation  accord- 
ingly assumes  that  in  Eozoic  times  all  animals  fed  on 
vegetable  foods.  Therefore  none  of  them  had  any  need  of 
defensive  structures,  and  they  remained  shell-less.  Then, 


The  Era  of  the  Dawn  of  Life 

just  before  the  beginning  of  Palaeozoic  times,  one  group  01 
organisms  is  supposed  to  have  become  carnivorous.  Its 
members  would  have  found  inexhaustible  supplies  of  food 
and  would  have  increased  apace.  They  accordingly  pro- 
ceeded to  devour  their  contemporaries,  and  to  avoid  this 
sudden  danger  all  sorts  and  conditions  of  animals  began 
to  develop  hard  'parts  for  self-defence.  Many  kinds  of 
animals  probably  failed  to  acquire  shells  or  skeletons,  and 
they  were  eaten  up  and  became  extinct.  This  hypothesis 
has  the  further  advantage  of  explaining  the  occurrence  of 
great  masses  of  Eozoic  limestones,  in  spite  of  the  absence 
at  that  time  of  shell-bearing  animals,  for,  according  to 
Prof.  Daly,  these  limestones  were  all  chemical  precipitates 
from  sea-water. 

There  are,  however,  two  serious  objections  to  this  in- 
genious hypothesis.  Many  hard  skeletal  structures  are 
developed  for  support  and  not  for  defence.  Thus  the  pink 
coral  used  for  jewelry  grows  as  an  internal  framework, 
which  is  surrounded  by  the  soft  tissues  of  the  coral  animal. 
The  live  corals  are  thereby  lifted  above  the  muddy  floor, 
and  by  the  branching  of  their  support  they  are  able  to 
collect  larger  quantities  of  food  and  oxygen,  just  as  the 
branching  of  trees  helps  them  to  collect  materials  from 
the  air.  Even  if  in  Eozoic  times  the  world  enjoyed  an 
Eden-like  peace  and  no  animals  chased  and  killed  others 
for  food,  yet  they  must  have  required  oxygen,  and  they 
would  have  secured  more  vegetable  food  had  they  been 
spread  out  over  a  structural  framework;  so  the  theory 
that  flesh-eating  began  with  the  Palaeozoic  Era  does  not 
explain  why  primitive  corals  and  sponges  did  not  build 
up  internal  skeletons. 

The  second  explanation  of  the  absence  of  calcareous 
structures  in  Eozoic  organisms  is  that  they  could  not  get 
carbonate  of  lime  however  much  they  may  have  wanted 
it.  The  seas  were  then  probably  smaller  and  shallower 

196 


The  Era  of  the  Dawn  of  Life 

than  they  are  to-day,  and  were  probably  more  thronged 
with  organisms.  In  existing  seas  the  zone  between  the 
well-lighted  upper  layer  and  the  layer  on  the  ocean  floor 
is  probably  relatively  poor  in  life.  At  one  time  many 
authorities  on  oceanography  considered  that  the  larger 
part  of  the  sea,  between  comparatively  thin  layers  on  the 
top  and  at  the  bottom,  was  practically  uninhabited ;  and 
though  that  view  has  been  disproved,  some  of  the  evidence 
which  led  to  it  indicates  that  the  intermediate  depths  are 
sparsely  occupied.  Marine  life  may  have  been  more 
evenly  as  well  as  more  densely  distributed  through  the 
shallow  Eozoic  seas  than  it  is  at  the  present  time ;  and 
the  decomposition  of  the  soft  parts  of  these  animals  would 
give  off  great  quantities  of  ammonium  carbonate.  This 
waste  product  would  react  on  the  sulphate  of  lime  present 
in  the  sea-water,  and  would  convert  it  into  carbonate  of 
lime,  which  would  then  be  thrown  down  as  a  fine-grained 
deposit.  Hence  there  would  have  been  a  smaller  propor- 
tion of  lime  in  solution  in  the  Eozoic  seas  than  there  has 
been  in  subsequent  periods ;  and  it  was  probably  not  until 
the  beginning  of  Palaeozoic  times  that  sufficient  lime  had 
accumulated  for  the  organisms  to  obtain  an  adequate 
supply  of  it  for  the  formation  of  hard  shells  or  skeletons. 
The  change  in  the  composition  of  the  sea- water  would 
have  taken  place  gradually,  so  that  it  is  not  surprising  that 
during  Cambrian  times  limestones  are  rare,  and  as  many 
of  the  most  abundant  of  Cambrian  animals  have  shells 
containing  phosphate  instead  of  carbonate  of  lime,  the 
occasional  Cambrian  limestones  are  often  rich  in  phos- 
phatic  material. 


197 


CHAPTER  XIII 
THE  FISH   ERA  :  PALAEOZOIC 

i.  THE  CAMBRIAN  SYSTEM. 

THE  Palaeozoic  rocks,  or  those  of  the  Era  of  Ancient  Life, 
are  the  first  that  retain  any  adequate  representation  of  the 
animals  which  were  living  on  the  earth  during  their  forma- 
tion. The  Palaeozoic  Group  is  divided  into  six  Systems, 
which,  in  order  of  time,  are  the  Cambrian,  Ordovician, 
Silurian,  Devonian,  Carboniferous,  and  Permian. 

The  Cambrian  System  derives  its  name  from  "  Cambria," 
as  its  rocks  were  first  studied  by  Adam  Sedgwick  in  North 
Wales.  As  a  rule  the  Cambrian  rocks  are  coarse-grained 
deposits  that  have  been  laid  down  in  shallow  water,  though 
these  are  interbedded  with  slates  and  shales  which  have 
yielded  most  of  the  fossils.  The  Cambrian  rocks  in  the 
British  Isles  are  best  known  in  Wales,  though  there  are 
outliers  in  the  English  Midlands,  where  they  include  a 
thick  series  of  altered  sandstones.  The  system  is  also 
represented  in  the  north-west  of  Scotland  by  similar 
sandstones,  and  some  shales  and  grits,  with  Cambrian 
fossils,  occur  on  the  southern  edge  of  the  Scottish 
Highlands. 

The  most  important  Cambrian  fossils  are  those  three- 
lobed  Crustaceans  known  as"trilobites,"and  as  they  were 
then  the  predominant  forms  of  life  in  the  world,  the  Cam- 
brian is  known  as  the  "  Age  of  Trilobites."  Its  oldest  fauna 
is  characterized  by  the  trilobite  Olenellus,  which  is  found 

198 


The  Fish  Era:    Palaeozoic 

in  the  lowest  Cambrian  rocks,  and  is  thus  the  type  fossil 
of  the  oldest  well-developed  fauna  that  has  yet  been  dis- 
covered. 

2.  ORDOVICIAN  SYSTEM. 

There  is  no  very  well-defined  boundary  between  the 
Cambrian  and  its  succeeding  System,  the  Ordovician,  and 
it  is  possible  that  another  System  may  have  to  be  adopted 
for  the  transitional  rocks  between  the  Cambrian  and  the 
Ordovician.  The  most  striking  feature  in  the  Ordovician 
Period  is  the  development  of  the  fossils  known  as  "grapto- 
lites  "  (Gr.,  graptos,  engraved  stone).  They  were  compound 
colonial  organisms,  living  in  plant-like  colonies,  and  are 
allied  to  the  living  sea-firs  (Sertularia),  and  less  closely  to 
the  jellyfish.  Each  graptolite  consists  of  a  large  number  of 
separate  individuals,  each  of  which  lives  in  a  little  horny 
cup.  The  essential  difference  between  the  graptolites  and 
the  living  sea-firs  is  that  each  branch  of  the  graptolite  is 
strengthened  by  a  horny  rod.  The  graptolites  appear  to 
have  lived  floating  on  the  sea,  and  in  some  forms  many 
branches  were  attached  to  a  central  float.  The  graptolites 
began  in  the  end  of  the  Cambrian  Period  and  survived 
into  the  Silurian  Period,  but  they  are  most  abundant  and 
most  characteristic  of  the  Ordovician,  which  is  therefore 
known  as  the  "  Age  of  Graptolites." 

The  Ordovician  was  a  Period  of  vast  and  widespread 
volcanic  activity,  and  probably  marked  a  time  during 
which  the  earth's  crust  was  convulsed  by  powerful 
mountain-forming  movements.  At  the  beginning  of  the 
Ordovician  Period  great  volcanoes  were  in  eruption  in  the 
Lake  District ;  while  in  the  typical  Ordovician  districts  of 
Shropshire  and  the  adjacent  parts  of  Wales,  shales  and 
quartzites  were  being  deposited  on  the  floor  of  a  quiet  sea. 
In  upper  Ordovician  times  the  volcanoes  in  the  Lake 
District  had  become  extinct,  and  a  fresh  centre  of  volcanic 

199 


The   Fish  Era :    Palaeozoic 

activity  had  burst  into  action  in  North  Wales,  where 
repeated  eruptions  built  up  the  volcanic  dome  of  Snowdon. 
That  the  climate  of  the  earth  as  a  whole  was  not  funda- 
mentally different  from  that  of  the  present  time  is  indicated 
by  the  size  of  the  particles  which  were  carried  by  the 
winds,  and  by  the  prevalent  wind  having  blown  across  the 
British  islands,  as  at  the  present  time,  from  the  south- 
west ;  for  the  beds  of  volcanic  ash  are  thicker  on  the 
north-eastern  side  of  Snowdon  than  they  are  on  the  south- 
western side,  which  was  then,  as  now,  to  windward. 

In  Ordovician  times  the  Southern  Uplands  of  Scotland 
were  covered  by  the  sea  ;  the  shore  was  then  near  Girvan 
on  the  Ayrshire  coast,  and  the  sea  extended  south-westward 
into  Ireland.  North-eastward  it  became  deeper,  so  that 
the  coarse  sandstones  and  conglomerates,  which  were 
formed  as  coast  deposits,  were  replaced  by  thin  layers 
of  fine-grained  black  shales. 

Most  of  the  Scottish  Highlands  must  then  have  been 
part  of  a  land  barrier,  which  extended  from  Scandinavia 
into  Ireland,  and  must  have  continued  much  farther  to  the 
south-west ;  for  the  seas  on  the  two  sides  of  it  were 
inhabited  by  distinct  faunas.  The  sea  to  the  north-west 
of  this  land  extended  from  New  York  to  north-western 
Scotland,  and  in  it  were  laid  down  the  thick  beds  of  lime- 
stone at  Durness  in  Sutherland.  These  and  the  thinner 
limestones  of  Bala  in  Wales  show  that  a  change  was 
taking  place  by  which  the  formation  of  limestones  was 
becoming  a  more  common  occurrence. 


3.  SILURIAN  SYSTEM. 

The  Silurian  System  forms  a  striking  contrast  to  its 
predecessor.  It  was  a  great  marine  period  during  which 
the  chief  geographical  event  was  a  gradual  advance  and 
retreat  of  the  sea.  The  coasts  and  lands  of  the  Ordovician 

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The   Fish  Era :    Palaeozoic 

Period  were  gradually  submerged.  This  change  was  due 
to  earth  movements,  which  appear  to  have  taken  place  so 
gently  that  they  did  not  occasion  volcanic  eruptions,  which 
are  very  scarce  and  local  in  the  Silurian,  though  they  occur 
in  some  areas,  as  in  the  Andes. 

The  typical  Silurian  rocks  are  shales  crowded  with 
fossils  and  great  lens-shaped  masses  of  limestone.  Some 
of  the  limestones  are  miles  in  length,  and  they  are  often 
so  crowded  with  corals  that  they  may  be  regarded  as 
fossil  coral  reefs.  The  best  known  of  the  British  Silurian 
limestones  is  the  Wenlock  Limestone,  which  forms  the 
ridge  known  as  "  Wenlock  Edge "  in  Shropshire.  The 
abundant  coral  growth  there  shows  that  the  sea  was 
warmer  than  it  is  on  the  British  coasts  at  present.  The 
sea  appears  to  have  become  colder  to  the  north,  for  the 
Silurian  rocks  in  the  Arctic  regions  contain  only  small 
nodules  of  corals  or  single  corals,  and  not  coral  reefs. 

The  Silurian  rocks  contain  beautifully-preserved  fossils, 
and  among  the  most  characteristic  are  the  fossil  sea-lilies, 
or  crinoids,  which  were  then  so  abundant  that  the  Silurian 
has  been  called  the  "  Age  of  Sea-lilies." 

The  most  remarkable  feature  in  the  life  of  the  Silurian 
was  the  appearance  of  the  first  fish,  which  are  the  oldest 
known  backboned  animals.  These  fish  are  represented  by 
their  teeth,  and  by  the  fin  rays,  which  are  the  hard  spines 
that  support  the  fins.  These  show  that  the  Silurian  seas 
swarmed  with  primitive  sharks. 

Towards  the  close  of  the  Silurian  Period  the  marine 
conditions  appear  to  have  been  gradually  replaced  by 
continental  conditions,  the  shales  pass  into  sandstones, 
which  were  laid  down  along  the  shore  and  contain  frag- 
ments of  drift-wood  and  the  remains  of  land-plants.  Thus 
the  Silurian  rocks  pass  upward  into  the  continental 
deposits  of  the  next  Period. 


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The  Fish  Era :    Palaeozoic 

4.  DEVONIAN  SYSTEM. 

The  Devonian  System  is  named  from  Devon,  where  its 
existence  was  first  recognized.  A  collection  of  fossils  was 
found  there,  and  Lonsdale  pointed  out  that  these  were 
intermediate  in  character  between  those  of  the  Silurian 
and  the  Carboniferous  Systems.  He  therefore  predicted 
that  a  group  of  rocks  would  be  found  intermediate  between 
those  Systems.  This  forecast  was  verified  by  study  of  the 
rocks  in  the  field. 

The  Devonian  System  includes  two  distinct  types  of 
deposits.  In  Devon  they  were  laid  down  in  the  sea. 
Both  the  Lower  and  Upper  Devonian  rocks  were  laid 
down  as  shallow  water  beds.  The  intervening  Middle 
Devonian  series  is  composed  of  shales,  and  interbedded 
with  them  are  thick  masses  of  limestone,  some  of  which 
were  formed  as  coral  reefs  on  the  shores  of  volcanic 
islands.  As  a  similar  development  of  limestone  between 
two  series  of  shallow  water  deposits  occurs  in  many  other 
parts  of  the  world,  as  far  distant  even  as  the  United  States 
and  southern  Australia,  it  appears  that  in  the  Middle 
Devonian  Epoch  there  was  a  world-wide  advance  of  the  sea 
upon  the  land.  This  rise  and  expansion  of  the  sea  would 
result  from  the  uplift  of  the  ocean  floors,  and  the  Middle 
Devonian  was  probably  a  Period  of  great  disturbances  of 
the  earth's  crust,  accompanied  by  the  elevation  of  moun- 
tain-chains and  the  shallowing  of  the  ocean  basins. 

The  continental  type  of  the  Devonian  is  known  as  the 
Old  Red  Sandstone.  It  consists  of  sandstones  and  shales, 
most  of  which  are  coloured  red.  They  contain  some  thin 
irregular  beds  of  limestone,  known  as  "cornstones,"  because 
the  soils  formed  from  them  are  good  for  corn-growing. 
The  limestones  were  probably  deposited  by  a  chemical 
process,  and  their  materials  were  formed  from  the  solution 
of  Silurian  limestones  on  the  adjacent  lands.  Some  of 

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The   Fish  Era :    Palaeozoic 

the  beds  in  the  Old  Red  Sandstone  Series  contain  many 
fossils,  but  these  are  chiefly  fresh-water  shells  or  ferns 
that  must  have  grown  on  land.  They  contain  many  fish 
which  may  have  lived  either  in  the  sea  or  in  fresh  water, 
for,  like  modern  fish,  they  probably  entered  the  rivers  to 
breed. 

There  is  a  complete  absence  from  the  Old  Red  Sand- 
stone of  typical  marine  organisms,  such  as  crinoids,  lamp- 
shells,  and  trilobites ;  so  that  the  nature  of  both  the  rocks 
and  their  fossils  shows  that  the  Old  Red  Sandstone  was 
not  formed  in  the  sea. 

The  Old  Red  Sandstone  is  found  in  Wales,  Scotland, 
Scandinavia,  north-western  Russia,  and  Spitsbergen.  It 
was  laid  down  on  a  great  continent,  the  southern  shore 
of  which  crossed  southern  Ireland,  the  south  of  Scotland, 
Belgium,  central  Germany,  and  western  Russia  south  of 
the  Gulf  of  Finland.  That  the  Old  Red  Sandstone  was 
contemporary  with  the  marine  Devonian  rocks  is  shown 
by  the  occurrence  of  the  same  fish  in  both  deposits,  and 
by  the  fact  that  in  Monmouthshire,  Herefordshire,  and 
South  Wales  the  Old  Red  Sandstone  occurs  between  the 
Silurian  rocks  and  the  Carboniferous.  The  two  series 
are  moreover  sometimes  found  interbedded.  The  Old 
Red  Sandstone  generally  occurs  in  great  isolated  areas, 
as  in  South  Wales  and  the  adjacent  border  counties,  in 
the  Midland  Valley  of  Scotland,  Argyll,  Caithness,  and 
the  Orkneys. 

Sir  Archibald  Geikie  was  led  by  his  classification  of  the 
deposits  to  assign  to  the  same  age  beds  which  contain 
very  different  fossil  fish.  He  explained  the  difference  in 
the  fish  found  in  the  supposed  contemporary  deposits  as 
due  to  their  having  lived  in  isolated  lakes.  He  therefore 
represented  the  British  Old  Red  Sandstone  as  having 
been  formed  in  four  great  lakes.  Lake  deposits,  however, 
are  usually  very  fine-grained  materials ;  for  lakes  consist 

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The  Fish  Era  :    Palaeozoic 

of  comparatively  stagnant  water ;  and  though  storms  may 
raise  a  heavy  surf  upon  the  shores  of  great  lakes,  they  are 
not  subject  to  the  constant  wash  to  and  fro  of  the  tide,  which 
is  the  most  powerful  factor  in  the  wear  of  beach  materials 
along  sea-coasts.  Lakes  are  great  settling  tanks  in  which 
the  fine  mud  carried  down  by  rivers  has  time  to  sink  to 
the  bottom;  and  though  the  rivers  may  deposit  coarse 
material  in  the  deltas  at  their  mouths,  the  typical 
deposits  on  the  floors  of  lakes  are  fine-grained  silts  and 
clays. 

The  Old   Red  Sandstone,  with   its   coarse  sands  and 
abundant  conglomerates,  was  mainly  formed  in  shallow 
water.     Some  of  the  pebble-beds  were  probably  formed 
on  land,  for  the  pebbles  have  been  cut  and  shaped  by 
wind-blown  sand.     The  Old  Red  Sandstone  was  probably 
formed  under  circumstances  similar  to  those  that  pro- 
duced  the  widespread   sheets  of  gravel  known  in  New 
Zealand  as  "shingle  rivers."     The  rivers  there  flow  down 
the  mountains  in  deep  gorges ;  when  they  reach  the  plains 
they  spread  out  and  deposit  vast   fan-shaped   sheets  of 
shingle.     The  rivers  flow  across  these  fans  along  shallow 
and  constantly  shifting  channels.     The  pebbles  are  left 
exposed  for  long  periods  to  the  action  of  the  wind  after 
the  river  has  shifted  its  course,  and  they  are  often  wind- 
polished  and  iron-stained.   Fish  swim  up  the  rivers  to  breed 
in  their  upper  waters,  where  the  young  are  exposed  to  fewer 
enemies  than  they  would  meet  in  the  sea.    The  shifting  of 
the  river  courses  would  occasionally  leave  bends  of  the  rivers 
isolated  as  lakes  or  pools.     These  would  gradually  dry 
up,  and  all  the  fish  would  be  thrown  down  together  in 
a  tangled  layer  on  the  floor  of  a  pool,  and  be  covered 
by  wind-blown   dust.     They  would  thus   form  fish-beds 
similar  to  those  which  are  the  most  important  fossiliferous 
deposits  in  the  Old  Red  Sandstone. 

The  Devonian,  unlike  the  Silurian,  was  a  great  volcanic 

204 


The   Fish  Era:    Palaeozoic 

period,  and  during  it  the  earth's  crust  was  disturbed  by 
one  of  the  greatest  epochs  of  mountain  formation. 

In  the  British  area  the  earth  movements  took  place 
mostly  in  or  at  the  end  of  the  Lower  Devonian ;  and  as 
the  Scottish  mountains  then  formed  had  a  trend  from 
north-east  to  south-west  (though  varying  between  from 
north-north-east  to  south-south-west,  and  from  east-north- 
east to  west -south -west),  this  trend  is  known  as  the 
"  Caledonian."  The  date  of  the  chief  Devonian  earth 
movements  is  shown  on  the  northern  side  of  the  Midland 
Valley  of  Scotland,  which  was  then  first  formed  or  enlarged. 
A  broad  belt  of  country,  fifty  miles  wide  and  extending 
across  Scotland  from  the  Firth  of  Forth  to  the  Firth  of 
Clyde,  was  slowly  lowered  between  the  Highlands  to  the 
north  and  the  Southern  Uplands  to  the  south.  The  lower- 
ing took  place  along  two  series  of  fractures  or  faults,  one 
of  which  forms  the  sharply  defined  boundary  between  the 
Highlands  and  Lowlands.  That  this  fault  was  formed 
after  the  Lower  Devonian  is  shown  by  the  fact  that  the 
Old  Red  Sandstone  beside  Loch  Lomond  has  been  thrown 
up  and  tilted  against  the  fault ;  while  the  main  movement 
was  completed  before  Upper  Devonian  times,  as  the  nearly 
horizontal  shales  of  the  Upper  Old  Red  Sandstone  were 
deposited  on  the  worn-down  edges  of  the  Lower  Old  Red 
Sandstone  and  across  the  fault. 

It  is  probable  that  the  movements  which  ended  with 
the  formation  of  these  great  faults  began  with  another 
type  of  earth  movement  which  disturbed  all  Scotland. 
In  the  north-western  Highlands  the  rocks  were  thrust 
north-westward,  and  some  old  rocks  were  pushed  over 
those  which  had  been  deposited  much  later.  In  Scotland 
the  direct  evidence  only  shows  that  these  overthrusts  took 
place  after  the  Lower  Ordovician ;  but  it  was  probably 
due  to  the  same  movements  as  those  which  buckled  and 
bent  the  Silurian  rocks  of  southern  Scotland.  This  move- 

205 


The  Fish  Era:    Palaeozoic 

ment  was  therefore  later  than  the  Silurian,  and  it  probably 
took  place  at  the  same  time  as  a  still  greater  overthrusting 
in  Scandinavia.  In  that  country  old  rocks  were  pushed 
eastward  over  younger  beds  for  a  length  of  1,000  miles ; 
and,  according  to  Prof.  HOgbom  and  other  Scandinavian 
geologists,  these  horizontal  movements  pushed  the  rocks 
sideways  for  a  width  of  about  150  miles,  though  this 
distance  may  have  been  over-estimated. 

These  great  Devonian  earth  movements  were  accom- 
panied by  renewed  outbursts  of  volcanic  activity.  The 
Ochil  Hills  and  the  hills  of  central  Argyll  were  built  up 
by  volcanoes  that  discharged  lavas  similar  to  those  of 
the  great  volcanoes  still  active  in  the  Andes.  Further 
evidence  of  the  disturbed  condition  of  the  earth's  crust 
in  Devonian  times  is  given  by  the  great  masses  of  granite 
which  were  then  forced  into  the  older  rocks.  These 
granites  may  have  been  the  roots  of  volcanoes,  all  traces 
of  which  have  been  swept  away  by  denudation. 

5.  THE  CARBONIFEROUS  SYSTEM. 

The  Devonian  Period  was  succeeded  by  the  Carbon- 
iferous, which  is  so  named  because  it  was  the  greatest  coal- 
forming  period  in  the  earth's  history.  In  many  parts  of 
the  world  coals  have  been  formed  in  later  geological  times ; 
but  the  chief  coalfields  of  Europe,  North  America,  and 
China  all  belong  to  this  System. 

The  Carboniferous  rocks  of  England  belong  to  two 
main  divisions.  The  typical  representative  of  the  lower 
division  is  a  thick  sheet  of  limestone  which  was  laid  down 
in  the  clear  waters  of  an  open,  but  shallow,  sea.  This 
limestone  was  formerly  called  the  "  Mountain  Limestone," 
because  it  forms  so  many  English  hills,  such  as  the 
Mendip  Hills,  the  Pennine  Range,  and  the  hills  beside 
the  coalfields  of  South  Wales  and  of  Flintshire.  The 

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The  Fish  Era:    Palaeozoic 

formation  is  now  generally  known  as  the  "  Carboniferous 
Limestone."  It  is  best  developed  in  the  south-west 
of  England.  There  the  shore  deposits  of  the  Upper 
Devonian  and  the  land  of  the  Old  Red  Sandstone 
gradually  sank,  and  were  covered  by  a  series  of  marine 
clays.  As  the  subsidence  continued  the  clays  were  suc- 
ceeded by  the  thick  series  of  limestones,  which  are  well 
shown  in  the  gorge  of  the  Avon  at  Bristol.  The  lower 
beds  of  the  Carboniferous  Limestone  contain  various 
fossil  corals  (Cleistopora  and  Zaphrentis)  that  are  not 
found  in  the  Midlands;  for  while  the  lower  limestones 
were  being  laid  down  in  the  Bristol  district,  the  old 
Devonian  land  still  existed  farther  north,  and  it  did  not 
sink  beneath  the  sea  until  the  time  of  the  higher  lime- 
stones of  Bristol. 

This  Midland  ridge  extended  from  northern  Wales  to 
southern  Yorkshire,  and  to  the  north  of  it  there  is  an 
unbroken  succession  of  beds  ranging  from  the  Upper 
Devonian  into  the  Carboniferous.  The  Carboniferous 
Limestone  is  no  longer  a  continuous  limestone,  but  is 
divided  into  separate  beds  by  layers  of  shale  and  sand- 
stone, which  proportionally  increase  to  the  north  and 
north-east.  Further  north,  in  the  east  of  Scotland,  the 
Lower  Carboniferous  is  represented  by  a  varied  series  of 
sandstones,  with  occasional  coal-seams  and  marine  lime- 
stones, and  it  includes  an  interesting  bed  of  fresh-water 
limestone ;  hence  the  district  around  Edinburgh  was 
generally  a  land  area,  though  it  was  occasionally  sub- 
merged beneath  a  shallow  sea.  In  western  Scotland  the 
Old  Red  Sandstones  are  succeeded  by  red  shales  and 
some  impure  limestones  ;  their  deposition  was  stopped 
by  a  volcanic  outbreak,  which  discharged  wide  sheets  of 
basalt  from  numerous  volcanic  vents.  They  built  up  the 
lava  plateaux  which  extend  from  Stirling,  past  the  north 
of  Glasgow,  to  the  hills  of  Renfrewshire.  After  the  close 

207 


The  Fish  Era :    Palaeozoic 

of  these  volcanic  eruptions,  western  Scotland  was  worn 
down  to  a  low  plain  covered  with  forests  ;  the  coal-seams 
were  accumulated  there,  while  all  southern  England  was 
under  the  Carboniferous  sea.  That  western  Scotland 
stood  but  slightly  above  sea-level  is  shown  by  the  sea 
having  repeatedly  flooded  the  land  and  deposited  thin 
sheets  of  marine  limestone.  At  the  close  of  the  time 
when  these  lower  coal-bearing  beds  were  being  formed  in 
Scotland,  and  the  Carboniferous  Limestone  was  being 
deposited  in  England,  the  whole  country  underwent  a 
marked  geographical  change,  which  led  to  the  formation 
of  the  sandstones  known  as  the  Millstone  Grit.  These 
represent  in  England  the  passage  from  the  Carboniferous 
Limestone  sea  to  the  land  conditions  of  the  English 
Coal  Measures.  In  the  west  of  Scotland  the  country 
was  covered  with  wide  salt-water  lagoons,  in  which  were 
laid  down  the  valuable  seams  of  fire-clay  worked  to  the 
east  of  Glasgow. 

During  the  upper  part  of  the  Carboniferous  Period  in 
Britain  the  geographical  conditions  were  more  alike  in 
England  and  Scotland.  Much  of  Britain  was  then  a  low- 
land, occasionally  flooded  by  the  sea.  The  climate  must 
have  been  warm  and  moist,  and  the  country  was  covered 
with  dense  forests,  and  the  accumulations  of  vegetable 
matter  on  the  ground  in  the  forests,  or  on  the  beds  of 
lakes  and  lagoons,  led  to  the  formation  of  valuable  coal- 
seams.  That  many  of  the  coal-seams  were  actually  laid 
down  in  the  forests  where  the  trees  grew  is  shown  by  the 
coal-seams  often  resting  on  beds  of  fireclay  that  served  as 
forest  soil,  for  the  roots  are  still  found  in  these  "  under- 
clays,"  and  the  stumps  of  the  trees  rise  up  from  the 
fireclay  into  the  overlying  seams  of  coal. 

In  the  shelter  of  these  forests  the  development  of  life 
made  another  great  advance.  In  Devonian  times  there 
were  fish  which,  like  the  modern  lung-fish  of  Queensland, 

208 


The   Fish  Era :    Palaeozoic 

acquired  the  power  of  living  through  periods  of  drought 
by  burrowing  into  the  mud  on  the  dried  river-bed  and 
breathing  air  through  a  lung.  Some  of  these  fish  appear 
to  have  left  the  shelter  of  their  mud  and  wriggled  about 
in  search  of  food  through  the  damp  jungle  on  the  floors 
of  the  forests.  This  exercise  led  to  an  increase  in  the  size 
and  power  of  the  limbs.  The  lung-fish  thus  gradually 
grew  into  amphibians,  the  class  now  represented  by  frogs, 
toads,  and  newts. 

The  Carboniferous,  therefore,  was  the  Period  during 
which  appeared  the  oldest-known  backboned  animals  that 
have  lived  on  land.  The  amphibians  had  developed  by 
the  Middle  Carboniferous  Series,  and  they  gave  rise  to 
reptiles  in  the  Upper  Carboniferous. 

The  true  Upper  Carboniferous  Series  is  not  the  same 
as  the  upper  part  of  the  Carboniferous  System  in  Great 
Britain,  for  the  third  or  uppermost  division  of  that  System 
is  barely  represented  in  the  British  Isles.  Its  typical  repre- 
sentatives are  in  eastern  Europe  and  Asia ;  it  is  also  well 
represented  in  Australia. 

At  the  close  of  the  deposition  of  the  British  Carbon- 
iferous rocks  the  country  underwent  a  series  of  great 
earth  movements,  which  led  to  the  formation  of  a 
mountain-chain  that  extended  from  central  Germany 
across  Belgium,  northern  France,  southern  England, 
and  southern  Ireland,  and  continued  far  westward,  where 
now  nothing  higher  than  stormy  billows  disturbs  the  wide 
level  of  the  Atlantic  Ocean.  This  mountain  system  is 
known  as  the  "  Armorican,"  after  the  ancient  name  ot 
Brittany,  where  it  probably  rose  higher  than  on  any  other 
part  of  western  Europe.  These  mountains  were  formed 
by  great  folds  due  to  the  land  to  the  south  of  it  having 
been  pressed  northward.  The  folds  trend  approximately 
east  and  west.  While  these  mountains  were  being  up- 
lifted, no  deposits  were  being  laid  down  in  southern 

209  o 


The  Fish  Era:    Palaeozoic 

England,  and  after  their  formation  they  were  rapidly 
denuded,  and  reduced  to  a  plain,  on  which  the  rocks 
of  the  next  System  were  laid  down. 


6.  THE  PERMIAN  SYSTEM. 

The  Permian  System  is  named  after  the  province  of 
Perm  in  eastern  Russia,  where  its  rocks  were  studied  by 
Sir  Roderick  Murchison.  The  Permian  System  in  eastern 
Europe  and  Asia  is  divided  into  three  series,  but  of  these 
the  lower  series  is  unrepresented  in  the  British  area  ,*  for 
it  was  the  time  of  the  culmination  of  the  earth  movements 
which  formed  the  Armorican  Mountains.  It  is  only  in 
the  Middle  Permian  that  fresh  deposits  were  formed  in 
north-western  Europe.  The  Middle  and  Upper  Permian 
are  represented  in  Britain  and  Germany  by  two  types  of 
deposits. 

Eastern  England  was  at  this  time  covered  by  an  inland 
sea,  which  extended  from  the  Pennine  Range  south-east- 
ward into  Germany.  That  its  waters  were  salt  is  shown 
by  its  abundant  marine  fossils;  but  that  the  water  was  of 
exceptional  composition  is  indicated  by  the  stunted  and 
deformed  growth  of  many  of  the  animals  that  lived  in  it, 
and  by  the  absence  of  many  groups  which  could  only 
flourish  in  normal  sea-water.  One  gulf  from  this  sea  ran 
west  of  the  Pennines  as  far  north  as  Cumberland,  and 
beyond  the  end  of  this  inlet  the  Permian  rocks  consist  of 
sands,  of  which  the  grains  are  often  so  beautifully  rounded 
that  when  a  slab  of  the  rock  is  examined  with  a  magnify- 
ing glass  it  looks  like  a  compact  mass  of  small  marbles. 
These  beautifully  rounded  and  polished  sand-grains  have 
clearly  been  formed  under  desert  conditions ;  and  that  the 
Penrith  Sandstone  was  laid  down  as  a  series  of  desert  sand- 
dunes  is  further  shown  by  the  irregularity  with  which  the 
successive  layers  were  piled  upon  one  another. 

210 


A  GROUP  OF  PERMIAN  REPTILES 

Some  Pariasaurus  are  climbing  on  to  the  boulders  to  avoid  the  Inostransevia,  who  is 
attacking  them  from  the  left.  The  Pariasaurus  grew  to  10  feet  long,  and  was  common  in 
South  Africa.  The  carnivorous  Inostransevia  was  first  discovered  in  Russia. 


The  Fish  Era :    Palaeozoic 

That  tidal  estuaries  ran  into  this  desert  land  is  indicated 
by  the  footprints  of  the  animals  that  lived  on  it.  These 
are  especially  well  preserved  in  the  quarries  of  Penrith 
Sandstone  at  Corncockle  Muir  and  other  localities  near 
Dumfries.  It  was  observed  that  the  footprints  were  all 
going  in  the  same  direction.  Sir  William  Jardine,  who 
described  these  fossil  footprints,  naturally  rejected  the 
explanation  suggested  to  him  that  they  were  made  by 
Scottish  reptiles  going  to  England.  He  explained  the 
constancy  of  direction  as  due  to  animals  having  fol- 
lowed the  retreating  tide  in  search  of  food,  and  thus 
left  their  footprints  when  the  ground  was  wet ;  but  when 
they  returned  before  the  rising  tide  they  left  no  im- 
pressions on  the  dry  ground. 

The  desert  period  of  the  Middle  Permian  was  probably 
due  to  the  Armorican  Mountains  having  extended  far  out 
into  what  is  now  the  North  Atlantic,  so  that  the  south- 
west winds  dropped  their  moisture  as  rain  upon  the  high- 
lands, and  swept  over  Britain  as  dry,  parching  winds. 

The  most  important  development  during  the  Permian 
Period  was  the  great  increase  in  the  number  and  variety 
of  the  reptiles.  They  are  best  known  from  the  fossils 
found  on  the  vast  land  which  then  occupied  a  large  part 
of  the  Southern  Hemisphere.  While  the  northern  forests 
consisted  of  the  giant  club-mosses  (Sigillaria  and  Lepido- 
dendron),  and  of  the  horse-tails  known  as  "  Calamites,"  the 
lands  of  the  Southern  Hemisphere  were  occupied  by  a 
quite  distinct  vegetation.  From  its  best-known  member 
it  is  described  as  the  "  Glossopteris  Flora."  Glossopteris 
(Gr.,  glossa,  tongue ;  pteris,  a  fern)  is  regarded  as  probably 
a  fern,  though  the  evidence  as  to  its  affinities  is  in- 
conclusive. 

The  Glossopteris  Flora  is  found  in  Australia,  India, 
southern  Africa,  and  South  America,  and  it  appears 
almost  certain  that  this  flora  could  only  have  ranged  from 

211 


The   Fish  Era :    Palaeozoic 

eastern  Australia  to  Brazil  if  there  had  been  a  continuous 
continent  between  those  distant  countries.  That  the 
plants  of  this  flora  did  not  migrate  eastward  and  westward 
across  the  lands  in  the  Northern  Hemisphere  is  indicated 
by  their  absence  from  the  northern  lands,  except  for  one 
colony  in  Russia.  Hence  the  distribution  of  this  flora  is 
evidence  of  the  existence  of  the  old  continent  of  Gond- 
wanaland,  which  included  the  Highlands  of  Brazil  on  the 
west  and  the  East  Australian  Highlands  on  the  east. 

Perhaps  the  most  interesting  deposits  of  Gondwanaland 
are  some  beds  of  hardened  boulder  clay,  which  show  that 
glaciers  existed  in  many  parts  of  this  ancient  continent. 
The  glacial  deposits  occur  in  southern  Brazil,  the  Argen- 
tine, South  Africa,  along  the  Irwin  River  in  western 
Australia,  in  Tasmania,  and  south-eastern  Australia. 
That  these  deposits  were  not  only  formed  on  high  moun- 
tains is  shown  in  western  Australia,  where  the  glacial 
beds  are  interstratified  with  marine  deposits,  and  also  in 
New  South  Wales,  where,  north  of  Sydney,  large  boulders 
brought  from  distant  localities  fell  through  the  water  and 
sank  edgewise  into  the  beds  on  the  sea-floor.  <  They  must 
have  dropped  from  melting  icebergs.  The  climate  of 
parts  of  the  Southern  Hemisphere  was  therefore  colder 
than  it  is  at  present. 


212 


CHAPTER  XIV 
THE  REPTILE  ERA:  MESOZOIC 

THE  Mesozoic  is  the  middle  Era  in  the  history  of  the 
world,  and  it  was  characterized  by  the  disappearance  of 
the  primitive  types  of  animals  and  plants.  Some  of 
the  ancient  groups  became  completely  extinct,  and 
others  survived  in  more  highly-developed  and  special- 
ized descendants.  Moreover,  during  the  Mesozoic  Era 
the  last  chief  groups  of  higher  animals  and  plants,  in- 
cluding mammals,  birds,  and  flowering  plants,  made  their 
first  appearance  on  the  earth. 

The  Mesozoic  Era  offers  a  striking  contrast  to  the 
closing  periods  of  the  Palaeozoic  by  its  quiet,  peaceful 
geographical  development.  After  the  crustal  storms 
which  upheaved  the  Armorican  Mountains  and  occa- 
sioned widespread  volcanic  eruptions,  the  earth  settled 
down  to  a  long  period  of  gentle  geographical  change.  The 
Mesozoic  Era  began  in  north-western  Europe  with  a  con- 
tinental period  which  continued  that  of  the  Permian,  and 
the  continent  was  finally  submerged  by  successive  ad- 
vances of  the  sea  upon  the  land.  The  submergence  was 
interrupted  about  the  middle  of  the  Era  by  a  temporary 
return  to  land  conditions  in  various  districts,  such  as  the 
south  of  England  ;  and  then  followed  another  long  period 
of  submergence,  which  ended  in  most  of  Europe  being 
drowned  beneath  the  deep  open  sea,  where  the  chalk  was 
being  deposited.  Then  the  long  Era  of  slow  change  sud- 
denly came  to  an  end.  The  sea-floor  was  raised  above 

213 


The  Reptile  Era:    Mesozoic 

sea-level,  and  this  movement  was  accompanied  by  great 
volcanic  outbursts,  during  which  vast  lava  sheets  were 
poured  forth  in  India,  Africa,  and  North  America.  The 
Mesozoic  is  therefore  an  Era  of  very  slow  geographical 
changes,  intervening  between  the  violent  crustal  disturb- 
ances which  closed  the  Palaeozoic  Era  and  those  which 
ushered  in  the  existing  Era. 


i.  THE  TRIASSIC  SYSTEM. 

The  Mesozoic  Group  is  divided  into  three  Systems. 
The  oldest  is  known  as  the  Triassic,  because  in  Germany, 
where  it  was  named,  it  consists  of  three  well-marked  sub- 
divisions. The  middle  division  is  a  marine  limestone ; 
the  upper  and  lower  divisions  consist  of  red  sandstones 
and  shales  deposited  on  land.  The  British  Triassic  rocks 
all  belong  to  the  red  kinds,  and  they  were  laid  down  under 
desert  conditions  on  land,  or  in  lakes,  or  in  lagoons. 
Both  the  land  and  its  climate  were  continued  from  those 
of  the  Permian  Period.  The  British  Trias  is  known  as 
the  "  New  Red  Sandstone  ";  it  is  called  "  new  "  to  dis- 
tinguish it  from  the  Old  Red  Sandstone.  Great  arms 
of  the  sea  extended  into  the  Triassic  land ;  these  were 
sometimes  completely  enclosed,  their  water  evaporated, 
and  the  mineral  matter  laid  down  in  beds  of  salt,  which 
are  worked  in  the  salt-mines  of  Cheshire  and  Worcester- 
shire. The  salt  is  obtained  by  pouring  water  into  the 
beds  and  allowing  it  to  dissolve  the  salt ;  the  brine  is  then 
pumped  up,  and  the  salt  obtained  by  evaporating  the 
water. 

The  clays  deposited  as  mud  on  the  floor  of  these  salt 
pools  contain  traces  of  the  creatures  that  lived  in  them, 
and  footprints  of  the  reptiles  that  roamed  about  their 
shores.  One  of  the  most  characteristic  fossils  from  these 
Triassic  beds  is  a  small  bivalved  Crustacean  (Estheria), 

214 


The  Reptile  Era :    Mesozoic 

which  still  lives  in  central  Australia  in  pools  that  last  for 
a  short  time  after  rain. 

The  Triassic  land  included  north-western  Europe  ;  to 
the  north  of  it  lay  an  Arctic  Ocean,  connected  with 
Triassic  Seas,  of  which  deposits  are  known  in  many  places 
around  the  Pacific.  Another  sea,  the  Tethys,  covered 
much  of  southern  Europe  and  parts  of  Asia,  and  formed 
the  beginning  of  the  present  Mediterranean. 

The  continent  of  Gondwanaland  was  still  in  existence, 
and  probably  still  extended  unbroken  from  Brazil  eastward 
to  Australia. 

2.  THE  JURASSIC  SYSTEM. 

The  Triassic  continent  of  north-western  Europe  was 
gradually  submerged  by  an  advance  of  the  sea  at  the 
beginning  of  the  next  or  Jurassic  Period,  so  named 
because  the  rocks  belonging  to  it  are  well  developed  in  the 
Jura  Mountains  of  north-western  Switzerland. 

The  Jurassic  began  with  some  passage  beds,  by  which 
the  red  shales  of  the  Trias  pass  gradually  through  red  and 
green  clays  into  black  clays.  Marine  fossils,  including 
bivalved  shells  and  the  teeth  and  scales  of  fish,  are  abun- 
dant in  the  black  clays.  This  marine  fauna  is  stunted, 
and  many  shells  are  distorted,  showing  that  the  sea  had 
not  yet  attained  its  ordinary  conditions.  With  further 
subsidence  of  the  land  the  outer  oceans  poured  into  the 
growing  inland  seas,  and  thus  the  succeeding  Liassic  clays 
were  laid  down  in  sea-water  of  normal  composition,  that 
teemed  with  well-developed  marine  animals. 

It  is  in  the  Lias  that  we  find  the  first  full  development  of 
Mesozoic  marine  life  in  north-western  Europe.  The  chief 
animals  without  backbones  were  Ammonites,  which  were 
shellfish  allied  to  the  living  Nautilus,  and  Belemnites,  which 
resembled  the  squids,  but  had  a  massive  central  axis  of 
limestone  instead  of  a  delicate  horny  "  pen." 

215 


The  Reptile  Era:    Mesozoic 

The  Lias  also  contains  many  skeletons  of  marine 
reptiles.  The  modern  dolphins  and  porpoises  are  fishlike 
mammals  which  have  descended  from  aquatic  quadrupeds ; 
and  the  Mesozoic  Ichthyosaurians  were  fishlike  reptiles, 
which  were  descended  from  the  land  reptiles  of  the  Permian 
by  gradual  adaptation  to  aquatic  life.  Among  other 
changes  the  legs  were  replaced  by  paddles,  and  the  tail 
was  provided  with  fins.  The  reptiles  were  abundant  and 
varied  in  form  and  mode  of  life  all  through  the  Mesozoic, 
which  is  therefore  known  as  the  "  Era  of  Reptiles,"  for 
they  were  supreme  alike  on  land,  at  sea,  and  in  the  air. 

The  Trias  has  yielded  the  first  remains  of  mammals,  but 
they  remained  small  and  scarce  throughout  the  Mesozoic. 
Many  lower  jaws  of  these  animals  have  been  found  in  two 
beds,  the  Stonesfield  Slate  of  Oxfordshire  and  the  Purbeck 
beds  of  Dorset,  both  of  which  belong  to  the  Oolitic  rocks. 
The  reason  why  only  the  lower  jaws  have  been  found 
is  that  the  bodies  were  doubtless  washed  out  to  sea,  and 
the  lower  jaws  soon  dropped  off  and  were  preserved  on  the 
sea-floor ;  while  the  rest  of  the  body  remained  floating, 
and  was  devoured  by  fish  or  reptiles.  These  mammals 
are  allied  to  those  still  living  in  Australia. 

The  Upper  Oolitic  beds  contain  the  oldest  known  bird, 
Archseopteryx  (p.  264). 

The  beds  containing  the  mammals  and  the  first  bird 
belong  to  the  Epoch  after  the  Liassic,  which  is  known  as 
the  "  Oolitic,"  from  the  prevalence  of  limestones  composed 
of  rounded  granules,  and  from  their  resemblance  to  the 
hard  roe  of  fish  they  are  described  as  "  Oolitic "  (Gr. 
oon,  egg;  lithos,  a  stone).  These  limestones  are  inter- 
bedded  with  clays  and  sandstones,  indicating  that  the  sea 
frequently  varied  in  depth  and  distance  from  the  shore. 
During  most  of  the  Oolitic  Epoch  nearly  all  England  was 
covered  by  the  sea,  while  to  the  north-west  there  must 
have  been  a  continent  from  which  the  sedimentary  material 

216 


THE  IGUANODON 

This  herbivorous  reptile  lived  in  the  Weald  of  Kent,  Belgium,  and  elsewhere.  It 
stood  15  feet  high.  The  thumb  was  altered  into  a  horny  spine,  projecting  at  right 
angles  to  the  palm  of  the  hand,  and  capable  of  inflicting  a  severe  wound. 


The  Reptile  Era:    Mesozoic 

was  obtained.  Estuarine  deposits  laid  down  at  the  mouths 
of  the  rivers  from  this  north-western  land  are  found  in 
Yorkshire  and  Lincolnshire;  and  the  Oolitic  rocks  of 
Scotland  show  the  prevalence  of  estuarine  and  terrestrial 
conditions,  for  among  them  normal  marine  beds  are 
exceptional. 

The  climate  at  the  time  must  have  been  warmer  than 
it  is  at  present,  for  some  of-  the  Oolitic  limestones  are 
so  crowded  with  reef-building  corals  that  they  have  been 
regarded  as  fossil  coral  reefs. 

The  last  British  deposits  of  the  Oolitic  are  found  in  the 
Isle  of  Purbeck,  where  they  contain  a  "  Dirt  Bed,"  which 
is  the  soil  of  a  former  forest.  The  stems  of  treefern-like 
plants  known  as  "  Cycads  "  are  found  still  standing  where 
they  grew,  and  with  their  roots  running  through  the  old 
soil.  A  fine  specimen  of  these  trees  may  be  seen  in  the 
fossil  plant  gallery  of  the  Natural  History  Museum  in 
London. 

3.  THE  CRETACEOUS  SYSTEM. 

The  Jurassic  Period  came  to  an  end  with  a  widespread 
upheaval.  The  British  area  was  again  converted  into 
land ;  and  this  lasted  into  the  lower  part  of  the  next 
System,  the  Cretaceous,  so  named  because  it  includes  the 
Chalk  (Lat.,  creta),  one  of  the  best-known  rocks  in  the 
British  Isles.  Its  white  (Lat.,  albus)  cliffs  gave  rise  to 
the  name  for  England  of  Albion.  The  lower  Cretaceous 
deposits  in  the  south  of  England  are  the  clays  and  sand- 
stones in  the  Weald  of  Kent,  Sussex,  and  Surrey.  The 
fossils  found  in  these  deposits  show  that  they  were  laid 
down  on  land  or  in  fresh  water.  They  were  probably 
formed  in  or  beside  the  estuary  of  a  great  river,  that 
appears  to  have  flowed  from  a  western  land  and  discharged 
into  the  ocean  then  covering  central  Europe. 

Amongst  other  fossils  the  Wealden  beds  have  yielded 

217 


The  Reptile  Era :    Mesozoic 

the  bones  and  teeth  of  many  gigantic  reptiles,  including 
the  great  Iguanodon  (p.  256).  This  Wealden  land  gradually 
sank  beneath  the  sea.  A  series  of  beds,  of  which  the 
most  characteristic  are  greensands,  were  formed  as  shallow 
water  deposits.  As  the  submergence  increased,  these  beds 
gradually  passed  into  the  pure  white  soft  limestone  known 
as  "  chalk."  This  rock  was  gradually  formed  of  the 
remains  of  innumerable  organisms,  which  lived  in  an  open 
sea  that  was  exceptionally  free  from  sediments.  The 
chalk  keeps  its  pure  calcareous  composition  all  across 
Europe  from  north-eastern  Ireland  to  the  Crimea.  The 
chalk  is  nearly  a  thousand  feet  in  thickness,  and  as  its 
deposition  was  going  on  continuously  and  almost  un- 
interruptedly, it  affords  many  most  instructive  examples 
showing  the  development  by  slow  and  gradual  stages  of 
one  species  of  animal  into  other  species. 


218 


CHAPTER   XV 
THE  MAMMAL  ERA:  KAINOZOIC 

THE  deposition  of  the  chalk  was  brought  to  an  end  by 
a  period  of  great  geographical  changes.  The  whole 
of  the  earth's  crust  appears  to  have  been  disturbed  by 
upheavals  that  raised  the  floor  of  the  Chalk  sea  into  land. 
These  movements  led  to  the  outbreak  of  one  of  the 
greatest  periods  of  volcanic  activity  in  the  history  of  the 
world. 

These  great  geographical  changes  inaugurated  the  Kain- 
ozoic  Era,  which  is  thus  called  as  its  animals  and  plants 
all  belong  to  recent  types  (Gr.,  kainos,  recent;  zoe,  life). 
For  some  reason,  still  imperfectly  understood,  the  great 
reptiles  that  lived  both  on  land  and  sea  in  the  Mesozoic 
Era  disappeared,  and  mammals  took  their  place  on  land, 
while  in  the  air  the  flying  reptiles  were  succeeded  by  birds. 
But  it  is  still  uncertain  whether  the  reptiles  were  extermi- 
nated in  the  struggle  for  existence  by  their  more  intelligent 
successors,  or  whether  the  mammals  and  birds  developed 
rapidly  and  occupied  the  place  rendered  vacant  by  the 
disappearance  of  the  reptiles.  The  mammals,  for  what- 
ever reason,  began  to  increase  in  number,  variety,  and 
size  at  the  beginning  of  the  Kainozoic,  so  that  it  is 
described  as  the  "  Era  of  Mammals." 

When  this  Era  began,  the  British  Isles  had  a  sub- 
tropical climate,  The  best-known  British  deposits  belong- 
ing to  the  Kainozoic  Group  are  in  the  south-eastern 
districts,  in  the  Thames  basin,  and  around  the  Isle  of 

219 


The  Mammal  Era :    Kainozoic 

Wight.  The  most  important  in  the  London  basin  is  the 
London  Clay,  which  was  deposited  in  a  shallow  sea  that 
extended  southward  to  southern  Belgium  and  into  northern 
France.  A  great  river  from  this  land  discharged  to  the 
sea  through  an  estuary  at  Sheppey,  which  is  an  island  at 
the  mouth  of  the  Thames.  This  river  flowed  through 
groves  of  palms,  whose  fossilized  fruits  may  be  found  in 
abundance  at  low  tide  on  the  mud  flats  to  the  north  of 
Sheppey,  along  with  the  bones  and  plates  of  crocodiles 
and  turtles  that  inhabited  the  river,  and  the  teeth  of  sharks, 
and  the  shells  of  sub-tropical  animals  which  lived  in 
the  sea. 

The  Kainozoic  deposits  generally  lie  in  isolated  basins, 
and  the  determination  of  the  relative  ages  of  the  beds  in 
the  different  basins  gave  great  difficulty,  until  Sir  Charles 
Lyell  introduced  the  method  of  correlation  by  the  per- 
centage of  living  species  of  shells.  According  to  the 
identifications  adopted  in  LyelPs  day,  about  3  per  cent,  of 
the  species  of  shellfish  found  in  these  Lower  Kainozoic 
deposits  are  still  living ;  so  he  grouped  these  beds  in  a 
system  which  he  called  the  "  Eocene  "  (Gr.,  eo,  dawn ;  kainos, 
recent),  or  "  the  dawn  of  recent  life."  In  other  Kainozoic 
beds  he  found  from  20  to  30  per  cent,  of  living  shellfish, 
so  he  called  them  the  "  Miocene,"  denoting  that  only  a 
minority  of  the  shells  belong  to  recent  species  (Gr.  melon  t 
less) ;  and  in  still  later  deposits  he  found  the  percentage 
of  living  species  amounted  to  between  36  and  95  per  cent.; 
so  he  called  this  division  the  "  Pliocene,"  or  period  full  of 
recent  life  (Gr.,pleion,  more;  kainos,  recent).  This  numerical 
method  was  successfully  employed  in  the  classification  of 
the  Kainozoic  Group.  Since  LyelPs  time,  however,  the 
numbers  he  used  have  been  altered,;  for  modern  students 
of  Eocene  shells  do  not  accept  any  of  them  as  identical 
with  living  species.  But  the  main  lines  of  the  classification 
established  by  Lyell's  method  is  still  adopted.  A  new 

220 


The  Mammal  Era :    Kainozoic 

System,  the  Oligocene,  has  been  introduced  between  the 
Eocene  and  Miocene  Systems,  and  the  Pleistocene  has 
been  added  for  the  System  after  the  Pliocene.  The  name 
of  this  last  division  has  given  rise  to  a  great  variety  of 
opinion.  Lyell  introduced  the  term  "  Pleistocene  "  for  the 
Upper  Pliocene,  but  Forbes  applied  it  to  the  beds  which 
Lyell  called  the  "  post-Pliocene."  Forbes'  use  of  the 
term  was  so  convenient  that  it  has  been  widely  adopted ; 
and  Pleistocene  is  therefore  generally  used  for  the  Period 
between  the  close  of  the  Pliocene  and  the  present  day. 

During  either  the  Eocene  or  Oligocene  Periods,  powerful 
volcanic  eruption^  broke  out  in  the  north-western  parts  of 
the  British  Isles;  they  formed  the  plateau  of  Antrim  in 
Ireland,  and  built  up  huge  volcanic  piles  around  five 
centres  in  north-western  Scotland.  The  islands  of  Skye 
and  Mull,  and  Ardnamurchan  Peninsula,  are  remnants  of 
these  volcanoes;  and  the  volcanic  area  extended  north- 
westward to  Iceland  and  Greenland.  The  age  of  these 
eruptions  is  still  doubtful.  They  are  associated  in  the 
Isle  of  Mull  with  some  plant  beds,  which  were  originally 
identified  as  Miocene.  The  most  conspicuous  plants  found 
in  these  beds  are  palm  leaves ;  and  as  the  Miocene  climate 
north  of  the  Alps  was  apparently  too  cold  for  palms,  the 
volcanic  eruptions  probably  happened  during  the  Eocene 
or  Oligocene  Periods.  The  uncertainty  regarding  their 
age  is  unfortunate ;  because  if  it  were  known  it  would  fix 
the  date  of  the  formation  of  the  North  Atlantic ;  for  the 
foundering  of  its  floor  was  probably  contemporary  with 
these  eruptions. 

Of  all  periods  of  mountain  formation  in  the  earth's 
history  the  Miocene  Period  was  perhaps  the  most  im- 
portant. Then  was  raised  the  greatest  of  modern  mountain- 
chains,  including  the  Alpine  System  of  Europe,  with  its 
branch,  the  Atlas  Mountains  in  Africa,  and  its  continuation, 
the  Himalayan  Mountain  System  of  Asia.  Then  also 

221 


The  Mammal  Era :    Kainozoic 

were  raised  the  Western  Mountains  of  North  America  and 
the  Andes  of  South  America.  The  mountain-forming 
movements  were  not  limited  to  the  Miocene,  for  they 
began  in  the  Oligocene  and  lasted  into  the  Pliocene.  It 
is  even  believed  that  the  Andes  and  the  Himalaya  are  still 
in  process  of  uplift.  The  Alps  are  chiefly  of  Miocene  date, 
but  their  representatives — the  Pyrenees  in  the  west  and 
the  Caucasus  in  the  east — were  both  mainly  upheaved  in 
the  Oligocene.  By  hidden  internal  forces  a  belt  of  the 
earth's  crust  was  thrust  northward ;  it  reared  itself  against 
the  old  lands  of  northern  Europe,  and  in  this  way  the 
Miocene  mountains  of  Europe  raised  their  snowy  peaks. 
In  Asia  the  movement  was  southward. 

There  are  no  marine  Miocene  deposits  in  the  British 
Isles,  for  the  country  was  being  uplifted  and  folded  at  that 
time,  and  therefore  denudation  and  not  deposition  was 
taking  place.  One  fold  was  formed  across  the  Weald  of 
Kent ;  the  North  and  South  Downs  are  remnants  of  the 
high  arch  formed  by  this  fold.  At  the  end  of  the  Miocene 
this  arch  and  other  highlands  were  worn  down  into  wide 
plains,  and  the  sinking  of  the  land  submerged  some  of  the 
eastern  coastlands  of  England  beneath  the  sea,  that  also 
covered  part  of  Cornwall.  The  eastern  sea  extended  on 
to  the  North  Downs,  and  some  shell  beds  there  show  that 
the  Wealden  ridge  had  been  planed  down  and  sunk  below 
the  sea-level  at  the  very  beginning  of  the  Pliocene  Period. 
The  best-known  Pliocene  beds  in  the  British  Isles  are  the 
Crags  of  Suffolk,  Norfolk,  and  north-eastern  Essex.  These 
are  a  series  of  beds  very  rich  in  shells  and  fragments  of 
those  plant-like  animals  known  as  the  "  Bryozoa  "  (Gr., 
brion,  moss ;  zoon,  an  animal). 

The  succession  of  the  fossils  in  these  beds  shows  that 
the  climate  was  becoming  colder.  It  appears  that  from 
the  time  of  the  London  clay  with  its  sub-tropical  con- 
ditions, the  British  temperature  must  have  been  falling 

222 


The  Mammal  Era:    Kainozoic 

steadily.  The  change  was  probably  due  to  a  different 
distribution  of  land  and  water,  whereby  currents  from  the 
south  were  excluded  from  the  British  area,  and  those  from 
the  Arctic  Ocean  were  exerting  growing  influence  on  the 
British  climate.  In  the  older  Crag  beds  there  are  many 
shells  of  sub-tropical  kinds,  which  had  probably  reached 
the  British  seas  from  the  Mediterranean.  They  gradually 
disappeared  in  the  higher  beds  of  the  Crag,  and  cold  water 
mollusca  increased,  until  in  the  upper  Crag  beds  there  are 
many  Arctic  shells,  and  warm -water  shells  have  com- 
pletely disappeared.  The  beds  which  succeed  the  Crags 
contain  the  remains  of  Arctic  animals  and  plants,  and  the 
deposits  themselves  were  clearly  laid  down  by  great  sheets 
of  ice. 

The  Pleistocene  System  begins  in  north-western  Europe 
as  a  series  of  irregular  drifts,  which  were  at  first  attributed 
to  a  great  flood.  The  interpretation  of  these  deposits  was 
the  most  important  of  the  Nature  stories  first  read  by 
Louis  Agassiz,  in  honour  of  whose  fiftieth  birthday  Long- 
fellow wrote : 

"  And  Nature,  the  old  nurse,  took 

The  child  upon  her  knee, 
Saying  :  '  Here  is  a  storybook 

Thy  Father  has  written  for  thee.' 

"  '  Come,  wander  with  me,'  she  said, 

'  Into  regions  yet  untrod  ; 
And  read  what  is  still  unread 
In  the  manuscripts  of  God.' " 

These  deposits  differ  from  those  laid  down  by  water,  as 
they  consist  of  tumultuous  masses  of  sand,  gravel,  and 
clay,  which  are  usually  unbedded  or  only  obscurely  bedded ; 
and  they  contain  large  boulders  of  rocks  brought  from 
distant  localities.  Many  of  the  boulders  are  flattened,  and 
the  smooth  faces  are  scratched ;  and  the  rocks  below  these 
deposits  have  been  constantly  ground  down  into  un- 

223 


The  Mammal  Era :    Kainozoic 

dulating  hummocks  with  grooved  and  scratched  surfaces. 
Louis  Agassiz  recognized  that  this  combination  of  char- 
acters is  also  found  in  the  beds  which  had  been  laid  down 
in  Switzerland  during  a  period  when  the  existing  Swiss 
glaciers  had  been  much  more  extensive.  So  he  attributed 
these  irregular  drifts  in  England  and  Scotland  to  former 
glacial  action. 

The  truth  of  this  view  was  soon  established,  and  it  was 
allowed  that  in  early  Pleistocene  times  most  of  the  British 
Isles  were  covered  by  sheets  of  ice.  The  scratching  of 
the  boulders  was  due  to  their  having  been  rubbed  together 
as  they  were  carried  forward  in  the  ice. 

That  the  clays  charged  with  these  boulders  had  not 
been  deposited  by  streams  of  water  was  clear,  as  no 
current  that  could  have  carried  such  large  masses  of  rock 
could  have  allowed  such  extremely  fine-grained  clay  to 
settle  in  the  same  place  as  the  boulders.  Though  the 
glacial  origin  of  the  material  was  soon  recognized,  there 
has  been  prolonged  controversy  as  to  whether  it  was  due 
to  land-ice  or  sea-ice.  In  the  Scottish  and  Welsh  valleys 
there  are  many  mounds  of  sand  and  boulders  identical  in 
character  with  the  moraines  formed  at  the  ends  of  the 
Swiss  glaciers;  and  there  can  be  no  doubt  that  these 
ancient  British  moraines  were  deposited  by  glaciers  that 
once  flowed  down  the  valleys.  On  the  floor  of  the  Mid- 
land Valley  of  Scotland,  and  on  the  plains  of  the  English 
Midlands  and  the  eastern  counties,  there  are  wide  sheets 
of  boulder  clay,  but  this  is  a  very  different  material  from 
that  found  in  ordinary  moraines. 

Moraines  consist  mostly  of  sand  and  boulders,  and 
usually  the  boulders  are  unscratched.  Mr.  Griffith  Taylor 
in  a  recent  Antarctic  Expedition  followed  one  moraine 
for  a  hundred  miles,  and  only  found  scratched  boulders 
in  one  locality.  "  On  these  slopes,"  he  says,  "  I  found  an 
ice-scratched  block,  the  only  specimen  I  had  seen  in  a 

224 


The  Mammal  Era  :    Kainozoic 

hundred  miles  of  moraine  debris  !"  In  many  localities 
the  boulder  clay  contains  marine  shells,  which  are  some- 
times in  fragments,  and  sometimes  are  numerous  and 
well  preserved.  Mr.  Joseph  Wright  of  Belfast  has  found 
foraminifera,  which  are  minute  marine  shells,  widely 
distributed  through  the  British  and  Irish  boulder  clays. 
Shell-bearing  clays  and  gravels  occur  to  the  height  of 
1,300  feet  above  sea-level  at  Moel  Tryfaen  in  North  Wales ; 
to  nearly  the  same  elevation  in  the  Midlands  of  England  ; 
to  about  1,000  feet  in  south-western  Scotland ;  and  to 
500  feet  above  sea -level  in  north-eastern  Scotland. 
According  to  many  students  of  glacial  geology,  these  shell- 
bearing  deposits  show  that  during  glacial  times  the 
British  Isles  were  at  a  lower  level  than  they  are  now ;  the 
sea,  therefore,  covered  the  lower  country,  and  the  boulders 
were  dropped  from  icebergs  and  ice-floes,  and  fell  into  the 
clay  on  the  sea-floor.  According  to  other  geologists,  how- 
ever, these  marine  fossils  were  carried  inland  by  glaciers 
in  lumps  of  frozen  mud,  and  the  boulder  clay  was  deposited 
by  the  melting  of  the  ice  ;  in  their  view  the  whole  of  the 
British  Isles,  north  of  a  line  from  the  mouth  of  the  Thames 
to  the  mouth  of  the  Severn,  was  once  covered  by  a  con- 
tinuous ice-sheet. 

There  is  no  general  agreement  on  this  question,  and 
opposite  views  have  been  expressed  by  the  highest 
authorities  on  the  subject.  Prof.  James  Geikie,  in  his 
"Great  Ice  Age"  (1894),  is  one  of  the  leading  upholders 
of  the  conclusion  that  the  glacial  deposits  were  all  due  to 
land  ice  ;  while  Prof.  Bonney  has  advocated  the  alterna- 
tive explanation  in  his  presidential  address  to  the  British 
Association  at  Sheffield  (1911).  There  are  serious  diffi- 
culties in  both  explanations.  The  balance  of  opinion  at 
present  is  in  favour  of  the  land-ice  theory,  though  it  fails 
to  explain  several  important  features  in  the  British  glacial 
deposits.  These  display  two  different  types.  In  the  hilly 

225  p 


The  Mammal  Era :    Kainozoic 

country  of  the  north  and  west  there  are  abundant  morainic 
deposits,  and  to  the  north  and  west  of  these  moraines  the 
ground  is  littered  with  boulders  and  morainic  heaps ;  but 
on  the  plains  to  the  south  and  east  there  are  wide  sheets 
of  boulder  clay  with  some  beds  of  gravel,  but  no  extensive 
moraines.  A  line  of  moraines  can  be  traced  across  the 
country  from  Flamborough  Head  through  Yorkshire  and 
along  the  eastern  margin  of  the  Welsh  uplands. 

These  moraines  must  have  been  deposited  along  the 
edge  of  a  great  ice-sheet,  which  covered  the  northern  and 
western  parts  of  the  country.  The  line  separates  two 
different  types  of  glacial  deposits,  the  plains  of  boulder 
clays  outside  the  moraines  and  the  hummocks  of  more 
sandy  irregular  deposits  within  the  moraines.  According 
to  Carvell  Lewis,  the  boulder  clay  was  laid  down  in  lakes 
outside  the  moraines  during  one  stage  of  the  glaciation. 
These  lakes  were  formed  by  rivers  which  were  flowing 
north-eastward,  and  were  dammed  up  by  the  margin  of 
the  great  ice-sheet.  The  natural  deposits  to  be  laid  down 
on  the  floor  of  such  lakes  would  be  fine  clay ;  most  of  its 
particles  would  be  derived  from  the  adjacent  beds,  and 
would  contain  ice-worn  boulders  of  rocks  from  distant 
localities ;  if  the  lakes  were  at  a  low  level,  they  might 
occasionally  be  flooded  by  the  sea  when  marine  organisms 
would  obtain  access  to  them.  In  some  districts,  as  in 
north-eastern  Essex,  the  boulder  clay  was  laid  down  by 
an  agent  which  had  a  great  horizontal  range,  but  a  very 
slight  range  in  height,  for  the  boulder  clay  lies  as  a  thin 
sheet  across  the  level  country,  and  the  hills  rise  through 
it  like  islands.  The  boulder  clay  lies  all  around  the 
bases,  but  none  of  it  was  deposited  on  the  hill-tops  or 
higher  slopes. 

It  is  true  that  farther  inland  the  level  of  the  boulder 
clay  rises ;  but  this  may  be  due  to  a  subsequent  uplift, 
as  farther  in  the  same  direction  the  glacial  shell- beds  are 

226 


The  Mammal  Era:    Kainozoic 

found  at  their  greatest  elevation.  The  essential  require- 
ments for  the  formation  of  boulder  clay  are  supplies  of 
ice-worn  stones,  of  local  de'bris,  and  of  ice-melting  under 
such  conditions  that  the  mud  deposited  is  protected  from 
the  flowing  water,  which  would  wash  away  the  very  fine 
particles  of  clay  and  leave  the  material  with  a  sandy 
instead  of  a  clay  base.  Such  conditions  occur  on  the 
floor  of  glacier  lakes,  in  quiet  estuaries,  and  where  sheets 
of  land-ice  are  melting  in  hollows  from  which  the  water 
finds  no  ready  escape.  Boulder  clay  is  being  formed  in 
Spitsbergen  where  the  glaciers  reach  the  sea,  and  this 
clay  is  rich  in  marine  shells,  so  it  may  be  a  marine  deposit. 
It  does  not  seem  possible  for  boulder  clay  to  be  formed  on 
land,  unless  in  positions  whence  the  water  set  free  by  the 
melting  ice  remains  practically  stagnant.  If  the  slope  is 
sufficient  to  give  the  water  a  strong  current  it  will  sweep 
away  the  particles  of  clay  and  leave  sand. 

The  one  place  where  I  have  seen  boulder  clay  being 
formed  on  land  was  at  the  head  of  the  Fulmar  Valley  in 
Spitsbergen,  during  Sir  Martin  Conway's  expedition  across 
that  Arctic  land.  We  there  had  to  traverse  a  basin  three 
miles  long  between  the  ice-cliffs  of  the  Ivory  Glacier  on 
one  side  and  the  snout  of  a  glacier  which  almost  com- 
pletely blocked  the  outlet  from  this  basin  on  the  other 
side.  Between  the  two  glaciers  was  a  tract  of  stony  bog, 
formed  of  soft  clay  charged  with  boulders  many  of  which 
were  ice-worn.  The  ground  was  so  soft  that  we  had  the 
greatest  difficulty  in  getting  our  ponies  through  it.  When 
that  ground  is  drained  it  will  contract  into  a  typical 
boulder  clay. 

It  appears,  therefore,  that  this  material  may  be  deposited 
either  on  land  or  in  the  sea,  and  the  origin  of  the  British 
deposits  must  be  determined  by  the  presence  or  absence 
of  sea-shells,  or  by  the  characters  of  the  associated  beds 
of  sand,  gravel,  and  bedded  clays. 

227 


The  Mammal  Era:    Kainozoic 

The  interpretation  of  the  British  glacial  deposits  is 
rendered  uncertain  by  the  doubt  whether  the  country  was 
ice-covered  once  or  more  than  once.  In  some  localities 
beds  containing  fossil  plants  and  bones  of  the  reindeer, 
mammoth,  and  other  animals  have  been  found  between 
layers  of  boulder  clay.  According  to  the  interpretation 
of  Prof.  James  Geikie  these  fossiliferous  beds  were  deposited 
during  intervals  of  a  mild  climate  between  distinct  glacia- 
tions.  He  holds  that  the  country  has  been  affected  by 
several  glaciations,  separated  by  mild  periods  when  the 
ice  disappeared,  except  from  around  the  mountain  summits. 
According  to  the  leading  authorities  on  the  glacial  geology 
of  the  Alps,  the  adjacent  lowlands  and  the  Alpine  valleys 
were  invaded  by  glaciers  at  four  periods,  and  these  were 
separated  by  non-glacial  periods.  These  glacial  periods 
are  classified  as  follows  by  Prof.  J.  Geikie  with  special 
reference  to  Britain  and  north-western  Europe,  and  by 
Profs.  Penck  and  Bruckner  for  the  Alps : 


Glacial  Periods  according 
to  Prof.  James 
Geikie. 

Alpine  Glacial  Periods 
according  to  Profs. 
Peuck  and  Bruckner. 

Fourth  Glacial     ... 
Third     Interglacial 
Third  Glacial 
Second  Interglacial 
Second  Glacial     ... 
First  Interglacial... 
First  Glacial 

Mecklenburgian 
Neudeckian 
Polandian 
Helvetian 
Saxonian 
Norfolkian 
Scanian  (not  in  Britain) 

Worm 

Third  Interglacial 
Riss 
Second  Interglacial 
Mindel 
First  Interglacial 
Giinz 

The  alternation  of  glacial  beds  with  those  deposited 
during  a  warm  climate  has  been  described  from  a  suburb 
of  Innsbruck,  where  two  boulder  clays  are  separated  by  a 
bed  containing  the  leaves  of  the  Pontic  rhododendron ; 
and  though  this  plant  grows  well  in  the  moister  parts  of 
Britain,  it  is  native  only  to  wet,  warm  districts  beside 
the  Mediterranean,  and  the  only  large  area  where  it  lives 

228 


The  Mammal   Era:    Kainozoic 

naturally  is  on  the  eastern  side  of  the  Black  Sea.  It  has, 
however,  been  claimed  by  other  geologists  that  this  plant- 
bearing  bed  at  Innsbruck  is  preglacial  and  belongs  to  the 
Pliocene,  and  therefore  gives  no  evidence  of  a  warm,  inter- 
glacial  period  in  the  Alps.  Still,  there  seems  little  doubt 
that  the  Alpine  glaciers  have  repeatedly  advanced  and 
retreated ;  but  in  Great  Britain  the  evidence  for  the  suc- 
cessive glaciations  and  the  interglacial  periods  is  more 
doubtful.  Mr.  Lamplugh,  one  of  the  leading  authorities 
on  the  British  glacial  deposits,  has  repeatedly  called  atten- 
tion to  the  absence  of  any  convincing  evidence  of  British 
interglacial  periods.  The  beds  containing  the  fossil  plants 
and  bones  have  been  exposed  in  temporary  excavations 
which  cannot  be  re-examined,  and  in  mine-shafts  where 
the  evidence  was  never  very  clear ;  they  do  not  occur  on 
the  long  cliff  sections  through  the  glacial  deposits  around 
the  coasts.  Mr.  Lamplugh  remarks1:  "With  all  this 
wealth  of  exposure,  we  might  reasonably  expect,  some- 
where on  the  coast,  to  have  found  definite  proof  of  the 
postulated  warm  interglacial  episodes ;  yet,  although  these 
hundreds  of  miles  of  cliffs  have  been  repeatedly  investi- 
gated, they  have  not  yielded,  so  far  as  I  am  aware,  a  single 
example  of  the  occurrence  between  the  boulder  clays  of 
a  deposit  containing  the  remains  of  a  contemporaneous 
flora  or  fauna  indicative  of  temperate  conditions,  or  of  any 
conditions  other  than  those  which  we  know  to  accompany 
the  borders  of  ice-sheets.  In  fact,  the  only  fossils  as  yet 
found  in  the  intercalated  beds  of  the  coast  sections  are 
the  fragmentary  derivative  shells  already  discussed  ;"  and 
he  concludes :  "  I  think  that,  even  on  theoretical  grounds, 
the  balance  tells  against  the  likelihood  of  more  than  one 
great  ice-sheet  having  covered  the  low  basins  of  north- 
western Europe  during  Pleistocene  times."2 

The  British  glaciers  must  have  made  many  local  advances 
and  retreats  during  the  Glacier  Period.   The  Alpine  glaciers 

229 


The  Mammal  Era :    Kainozoic 

sometimes  increase  in  one  valley  while  they  are  diminishing 
in  another;  and,  according  to  Mr.  Lamplugh,  the  ice  in 
the  British  Isles  may  have  undergone  many  similar  local 
oscillations,  but  he  holds  that  these  were  not  simultaneous. 
The  occurrence  of  plant-beds  between  boulder  clays  is  no 
proof  of  a  warm  interglacial  period,  for  fir-forests  with 
dense  undergrowths  live  on  some  of  the  Alaskan  glaciers, 
and,  owing  to  the  continuous  daylight,  the  foliage  on 
Arctic  plants  grows  during  the  summer  with  remarkable 
luxuriance.  The  plants  growing  on  the  moraines  on  the 
Alaskan  glaciers  will  be  doubtless  buried  among  the  glacial 
deposits,  and  such  an  occurrence  is  therefore  no  proof  of 
an  interglacial  episode. 

The  length  of  time  which  has  elapsed  since  the  British 
glaciation  is  uncertain.  The  topography  of  England  now 
is  practically  the  same  as  it  was  when  the  Romans  arrived 
nearly  2,000  years  ago;  for  the  Roman  fords,  camps,  and 
drains  show  that  the  valleys  had  been  even  then  cut  down 
to  their  present  levels.  There  have,  however,  been  great 
geographical  changes  since  glacial  times.  Many  of 
the  smaller  Essex  valleys  have  been  entirely  cut  out 
since  the  deposition  of  the  boulder  clay ;  and  there 
is  evidence  in  all  parts  of  the  country  of  great  post- 
glacial changes  in  the  form  of  the  land.  Hence  it  seems 
reasonable  to  believe  that  many  times  the  length  of  the 
post-Roman  interval  must  have  elapsed  between  glacial 
and  Roman  times.  It  is,  moreover,  agreed  that  man 
arrived  in  central  Europe  and  southern  France  either  in 
the  second  interglacial  or  third  interglacial  period,  and 
that  he  occupied  England  after  the  Wurm  glaciation,  and 
while  glaciers  still  existed  in  Scotland  ;  and  as  these  early 
British  men  probably  lived  many  tens  of  thousands  of 
years  ago,  the  Glacier  Period  in  this  country  came  to  an 
end  at  a  date  which  must  be  very  remote  when  judged  by 
historic  standards. 

230 


The  Mammal  Era:    Kainozoic 

In  Scandinavia  Baron  de  Geer  has,  however,  recently 
maintained  that  the  glaciers  covered  the  whole  of  Sweden 
only  12,000  years  ago,  and  covered  Sweden  as  far  south 
as  Stockholm  and  Lakes  Wener  and  Wetter  only  9,000 
years  ago.  If  the  British  glaciation  were  simultaneous 
with  that  of  Scandinavia,  then  our  Glacier  Period,  accord- 
ing to  Baron  de  Geer's  chronology,  would  have  lasted 
until  less  than  10,000  years  ago,  and  this  is  incredible. 
It  is  by  no  means  certain  that  the  glaciations  of  the 
different  European  areas  were  synchronous.  In  North 
America  the  great  ice-sheets  developed  around  three 
different  centres  at  successive  dates.  First  came  the 
glaciation  of  the  western  mountains  of  Canada  and  north- 
western mountains  of  the  United  States  ;  these  western 
glaciers  flowed  eastward  over  the  plains  of  central  Canada 
to  the  Great  Lakes.  As  this  ice-sheet  waned,  a  second 
glaciation  developed  on  the  highland  of  Labrador  in 
eastern  Canada,  and  its  glaciers  flowed  south-westward 
to  the  Great  Lakes  and  southward  to  New  York.  As  this 
ice-sheet  disappeared,  a  third  glaciation  developed,  and 
now  covers  Greenland ;  and  it  may  have  been  formed  so 
recently  that  the  Greenland  glaciers  are  possibly  not  yet 
at  their  maximum.  Some  geologists  argued  that  at  the 
time  of  the  European  glaciation  the  Greenland  ice-sheet 
must  have  been  much  greater  than  it  is  now,  and  that 
it  spread  over  all  the  coast -lands  of  Greenland  and 
extended  across  the  sea  to  the  islands  north  of  America. 
But  if  so,  the  ice  would  have  exterminated  all  the  land 
plants,  and  when  the  coast-lands  of  southern  Greenland 
were  again  left  bare,  they  would  have  been  occupied  by 
plants  from  America ;  whereas  the  present  flora  is  Scandi- 
navian in  origin,  and  dates  from  a  time  when  Greenland 
was  connected  by  continuous  land  with  north-western 
Europe  and  was  isolated  from  America.  Most  of  the 
authorities  who  have  studied  the  Greenland  glaciers  agree 

231 


The   Mammal  Era:    Kainozoic 

that  much  of  the  open  land  around  the  coasts  has  never 
been  covered  by  ice,  for  it  is  so  rough  and  rugged  that 
it  has  not  been  worn  down  by  ice.  The  glaciers  have 
been  more  extensive  locally,  but  both  on  the  eastern  and 
western  coasts  long  tracts  of  coast-land  show  no  trace  of 
ice-action. 

It  is  therefore  possible  that,  just  as  the  eastern  glacia- 
tion  of  North  America  was  later  than  that  in  the  Western 
Mountains,  the  glaciation  of  Scandinavia  may  have  been 
much  later  than  that  of  Britain  ;  and  so  Palaeolithic  man 
may  have  lived  in  England  while  glaciers  still  covered 
the  mountains  of  Scotland,  and  long  before  the  Scandi- 
navian ice-sheet  had  reached  its  maximum. 

1  G.  W.  Lamplugh,  "The   Interglacial   Problem  in  the   British 
Isles"  (paper  read  at  the  International  Geological  Congress,  1913), 
p.  4. 

2  Ibid.,  p.  6. 


232 


PART  IV 
THE  STORY  OF  LIFE  ON  THE  EARTH 

CHAPTER  XVI 

THE  ORIGIN  OF  LIFE 

THE  story  of  life  on  the  earth  is  one  of  the  most  generally 
attractive  branches  of  geology.  To  former  life  products 
we  owe  materials  indispensable  to  our  comfort  and  even 
to  our  existence  and  civilization.  Many  of  the  most  useful 
minerals,  such  as  limestone  which  provides  cements  and 
building-stones,  coal,  oil,  and  other  fuels,  and  some  iron 
ores,  have  been  produced  by  former  animals  or  plants. 

As  to  the  origin  of  life,  geology  gives  little  direct 
evidence.  Our  knowledge  of  the  living  occupants  of  the 
early  world  depends  on  fossils,  which  are  represented  by 
the  hard  parts  of  animals  or  plants  that  have  been  pre- 
served in  the  rocks,  by  casts  and  by  impressions  left  on  soft 
mud  by  footprints,  by  leaves,  by  plant-stems,  or  by  soft 
animals,  such  as  jelly-fish ;  but  fossil  soft-bodied  animals 
are  quite  exceptional.  The  history  of  life  in  the  world 
is  therefore  mainly  limited  to  those  animals  and  plants 
which  had  hard  skeletons,  shells,  or  thick  durable  stems. 
The  larvae  of  animals  and  often  the  young  also  are  soft- 
bodied  ;  the  hard  structures  are  developed  or  strengthened 
later  in  life,  and  probably  the  earliest  animals  were  all 
soft-bodied.  Hence  we  cannot  expect  to  find  fossil 
remains  of  the  earliest  forms  of  life. 

233 


The  Origin  of  Life 


Geology  helps  the  solution  of  the  difficult  problem  of 
the  origin  of  life  by  indicating  the  conditions  under  which 
life  first  appeared  upon  the  globe.  The  first  appearance 
of  life  on  the  earth  must  have  been  far  back  in  the  Eozoic 
Era.  In  one  stage  of  that  Era  the  earth  probably  pro- 
duced some  living  material  which  has  acted  as  the 
progenitor  of  all  the  animals  and  plants  that  have  since 
lived  upon  the  earth.  It  has  often  been  suggested  that 
life  was  introduced  to  this  planet  from  some  other  heavenly 
body ;  but  it  appears  more  probable  that  the  life  on  the 
earth  is  one  of  its  own  products.  It  was  long  held  that 
living  and  non-living  matters  are  separated  by  a  barrier 
which  could  only  be  crossed  by  a  direct  act  of  creation. 
Huxley,  in  a  famous  essay,  showed  that  this  view  was 
unnecessary ;  but  the  probable  steps  of  the  process  by 
which  dead  matter  has  passed  into  living  matter  have  only 
been  recognized  in  recent  years. 

If  there  be  any  absolute  difference  between  living  and 
non-living  matter  it  should  be  capable  of  definition.  But 
all  the  definitions  of  life  and  of  vitality  apply  to  the  more 
complex  forms  of  crystal  growth. 

The  distinction  between  living  and  non-living  matter 
appears  to  be  so  indefinite  that  it  is  usually  based  on 
a  statement  of  the  general  properties  of  living  matter.  At 
present  the  higher  forms  of  animals  and  plants  have 
several  powers  which  non-living  matter  does  not  possess. 
Animals  can  defend  themselves  against  attack  ;  they  can 
adapt  themselves  to  changes  in  external  conditions  ;  and 
they  have  an  intelligence  based  on  memory  of  former 
experiences. 

If  we  compare  even  the  more  primitive  of  these  intelli- 
gent, adaptable,  defensive  organisms  with  even  the  most 
complex  of  crystals,  the  difference  between  them  appears 
fundamental.  And  if  life  had  begun  on  the  earth  with 
highly-organized  animals  and  plants,  it  would  be  necessary 

234 


The  Origin  of  Life 


to  assume  their  sudden  creation.  Life,  however,  probably 
began  in  something  which  had  a  far  simpler  organization 
than  any  existing  living  substance. 

At  the  present  time  the  simplest  type  of  living  matter 
is  the  cell,  which  is  really  a  very  complex  and  highly- 
specialized  structure.  It  consists  of  a  speck  of  "  proto- 
plasm," which  is  enclosed  in  a  cell-wall,  and  contains 
a  nucleus.  The  nucleus  has  a  complex  structure  and 
mysterious  properties.  According  to  some  biologists  the 
nucleus  is  the  essential  part  of  the  cell,  and  some  cells  are 
said  to  consist  of  the  nucleus  alone.  Other  biologists 
insist  that  the  protoplasm  is  the  essential  part.  And  this 
view  urged,  e.g.,  by  Dr.  C.  Walker,  appears  inherently  the 
more  probable.  Life  doubtless  began  with  some  organism 
which  was  far  simpler  than  the  cell,  and  consisted  of 
protoplastic  matter  devoid  of  wall  or  nucleus,  and  was 
a  mere  patch  of  carbonaceous  jelly. 

The  most  striking  difference  in  chemical  composition 
between  living  and  non-living  materials  is  that  the  former 
consist  of  carbon  combined  with  the  gases  oxygen,  hydro- 
gen, and  nitrogen,  and  contain  minute  proportions  of  a 
few  other  constituents.  Organic  or  living  bodies  are 
essentially  carbonaceous  in  composition.  The  non-living 
materials  are  mainly  compounds  of  silica,  iron,  calcium, 
and  other  elements. 

Though  geology  gives  little  direct  evidence  as  to  the 
geographical  conditions  at  the  time  of  the  origin  of  life,  we 
may  reasonably  infer  that  the  earth  was  fairly  warm.  It 
cannot  have  been  hotter  than  about  150°  to  160°  F.,  for 
higher  temperatures  are  injurious  or  fatal  to  many  organic 
processes.  The  atmosphere  of  the  earth  was  probably 
charged  with  moisture,  and  the  whole  sky  covered  with  a 
heavy  pall  of  thick  clouds.  Hence  the  temperature  on  the 
earth's  surface  would  have  been  almost  uniform  day  and 
night.  The  steamy  atmosphere  would  have  contained 

235 


The  Origin  of  Life 


various  phosphides,  and  chlorides  and  other  gases  which 
have  long  since  been  removed  from  the  air.  On  the 
shores  of  the  Eozoic  Sea  there  would  have  been  pools 
of  warm  water,  and  the  soft  mud  beside  them  would  have 
served  as  an  excellent  medium  for  the  support  of  the  first- 
formed  organism.  The  conditions  beside  these  pools 
would  have  been  constant  throughout  the  year,  so  there 
would  have  been  no  need  for  the  organism  to  have  had  the 
power  of  adaptation  to  changes  in  external  condition.  It 
would  have  had  no  need  for  defence  against  other 
organisms,  and  it  would  have  had  no  intelligence,  as 
it  had  nothing  to  remember. 

The  name  "  Protobion  " — i.e.,  the  first  living  being,  has 
been  suggested  for  the  name  of  the  first  organism.  It 
would  have  been  a  carbonaceous  body  which  differed 
from  non-living  material  by  having  the  powers  of  feeding, 
growth,  and  reproduction.  The  first  stage  in  the  evolution 
of  this  organism  would  have  been  the  formation  of  carbo- 
hydrates ;  these  compounds  are  so  named  because  they 
consist  of  carbon  combined  with  hydrogen  and  oxygen  in 
the  proportions  present  in  water.  Starch  (C6H10O5)  is  a 
carbohydrate,  because  it  has  the  same  composition  as 
a  mixture  of  six  parts  of  carbon  with  five  of  water.  The 
carbohydrates  are  not  formed  by  adding  carbon  to  water, 
but  by  the  combination  of  carbon  dioxide  (CO2)  with 
water  (H2O).  Thus  starch  is  formed  by  six  parts  of  carbon 
dioxide  combining  with  five  parts  of  water,  with  the  separa- 
tion of  the  excess  of  oxygen.  Thus : 

6CO2  +  5H2O  =  C6H10O6  -I-  6O2. 

This  process  can  only  go  on  in  the  presence  of  some 
source  of  energy,  such  as  sunlight  or  an  electric  discharge. 
The  carbohydrates,  though  usually  formed  by  plants,  have 
been  made  artificially. 

The  second  stage  in  the  evolution  of  life  is  the  combina- 

236 


The   Origin  ot  Life 


tion  of  nitrogen  with  the  carbohydrate  ;  and  a  simple 
nitrogenous  carbohydrate  is  converted  by  the  weaving  of 
its  particles  into  the  more  complex  substance  protein, 
which  is  the  essential  nitrogenous  constituent  of  animal 
and  plant  tissues.  Protein  was  named  in  1838  from  a 
Greek  word  proteion,1  or  primary,  as  it  is  the  primary 
material  in  the  bodies  of  animals  and  plants. 

There  is  no  chemical  difficulty  in  the  formation  of  the 
essential  constituents  of  living  beings  by  purely  chemical 
processes,  but  a  protein  would  not  be  alive  unless  it  had 
the  powers  of  growth,  of  absorbing  food,  and  of  reproduc- 
tion. It  was  long  held  that  animals  grew  from  within  by 
the  internal  assimilation  of  food,  and  that  minerals  grew 
by  the  addition  of  layers  on  the  outside.  This  distinction 
is  no  longer  valid.  Many  purely  artificial  mineral  bodies 
have  the  powers  of  growth  by  internal  assimilation. 

The  essential  property  which  separated  Protobion  from 
mere  specks  of  chemically  formed  protein  is  that  after  it 
had  grown  to  an  inconvenient  size  it  would  subdivide  into 
smaller  bodies,  each  of  which  had  the  power  of  growth 
and  reproduction.  This  process  of  reproduction  can  be 
most  simply  explained  as  due  to  the  action  of  one  of  those 
agents  known  as  "  catalysers."  A  catalyser  is  an  agent 
which  starts  an  operation  without  appearing  to  take  any 
part  in  it.  It  acts  like  a  trigger  in  setting  other  forces  in 
action  ;  a  minute  trace  of  a  catalyser  may  affect  an  enor- 
mous amount  of  material.  Turning  the  tap  of  a  reservoir 
takes  just  the  same  energy  to  set  in  motion  a  pint  of  water 
as  to  start  the  flow  of  a  million  tons.  Many  of  the  appar- 
ently most  mysterious  vital  processes  are  now  known  to 
be  due  to  the  action  of  catalysers.  It  is  the  presence  of 
the  catalyser  "  diastase  "  which  enables  animals  to  convert 
starch  into  sugar  ;  and  the  influence  of  the  nucleus  in 
starting  the  subdivision  of  living  cells  is  probably  due  to 
some  phosphoric  catalyser  which  is  present  in  the  nucleus. 

237 


The  Origin  of  Life 


It  was  possibly  by  the  addition  of  a  phosphoric  catalyser 
to  some  primitive  inorganically  formed  protein  body  that 
gave  it  the  power  of  continuous  self-division,  and  thus 
converted  some  globules  of  inorganic  protein  into  the 
living  Protobion.  The  loss  of  water  from  the  external 
layer  would  have  given  the  organism  an  enclosing  envelope, 
and  the  concentration  of  the  catalyser  into  a  central  par- 
ticle would  have  led  to  the  formation  of  a  nucleus.  Thus 
Protobion  would  have  passed  into  the  first  cellular  or- 
ganism.2 

1  This  derivation  is  given  in  Watts'  "Dictionary  of  Chemistry"; 
the   word  also  means  "  pre-eminence."     "  Protein "  has   been   ex- 
plained as  derived  ;from  protos,  first,  and  the  suffix  in,  and  explained 
as  holding  the  first  place  among  albuminous  principles. 

2  This  theory  of  the  origin  of  life  was  issued  independently  and 
simultaneously  by  Sir  Edward  Schafer  in  his  Presidential  Address 
to  the  British  Association,  and  by  the  author  in  "  The  Making  of  the 
Earth"  (Home  University  Series,  1912).    The  two  views  differed 
mainly  in  that  Sir  Edward  Schafer  regards  the  formation  of  life  from 
non-living  matter  as  having  probably  taken  place  continuously,  and 
as  perhaps  still  happening,  whereas  the  author  regarded  it  as  limited 
to  one  period  in  the  early  history  of  the  world  under  special  atmo- 
spheric and  geographical  conditions.     A  series  of  interesting  articles 
on  the  same  subject  appeared  in  Science  Progress,  1912  and  1913,  by 
Prof.  Armstrong,  Prof.  Minchin,  Dr.  C.  Walker,  and  others.     Prof. 
Armstrong  (Science  Progress,  No.  26,  1912)  also  holds  that  life  was 
produced  in  an  early  geological  period,  and  is  not  now  being  made 
from  non-living  matter. 


238 


CHAPTER  XVII 

THE   INTERPRETATION   OF   FOSSILS  :   FOSSIL 
FISH  AND  AMPHIBIANS 

THE  geologist  adopts  the  division  of  animals  into  two 
groups — the  Vertebrates,  or  animals  with  backbones,  and 
the  Invertebrates,  which  have  no  backbone.  The  Inver- 
tebrates have  shells,  or  internal  skeletons  like  the  corals, 
or  the  only  hard  parts  may  be  jaws,  as  in  some  worms. 
The  chief  kinds  of  Invertebrates  are  as  follows  : 

Protozoa. — Primitive  animals,  with  usually  microscopic  shells — 

the  Foraminifera  and  Radiolaria. 

Ccelenterata.  —  Sponges,    sea  -  firs,    graptolites,    jelly-fish,    sea- 
anemones,  corals,  etc. 
Echinoderms. — Sea-lilies,    sea-urchins,   star-fish,   sea-cucumbers, 

etc. 

Vermes. — Worms.  i 

Arthropods. — Crabs,  shrimps,  trilobites,  sand-hoppers,  wood-lice, 

scorpions,  insects,  etc. 
Molluscoidea. — Moss-animals  (Bryozoa)  and  lamp-shells  (Brachio- 

pods). 
Mollusca. — Shell-fish,  squids,  octppus,  etc. 

\ 

The  Invertebrates  furnish  the -most  numerous  fossils,  and 
historic  geology  is  mainly  dependent  on  their  evidence. 
They  are  also  geologically  important  as  rock-builders. 
They  are  more  primitive  in  structure  than  the  backboned 
animals,  and  began  to  live  on  the;  earth  much  earlier.  In 
fact,  all  the  chief  groups  of  marine  invertebrates  were 
already  in  existence  at  the  beginning  of  the  Palaeozoic 
Era.  The  backboned  animals  do  not  appear  until  the 

239 


The  Interpretation  of  Fossils 

Silurian  Period,  and  they  therefore  have  the  great  added 
interest  to  the  geologist,  that  he  can  trace  the  origin  and 
development  of  all  their  classes.  The  early  history  of  the 
invertebrates  has  been  lost  for  ever. 

Geology  throws  no  definite  light  on  the  origin  of  the 
backboned  animals  because  they  were  descended  from  a 
soft-bodied,  worm-like  animal.  According  to  Dr.  Gaskell, 
the  Vertebrates  are  the  offspring  of  the  Eurypterids,  which 
were  an  extinct  class  allied  to  the  Crustaceans.  Dr.  Smith 
Woodward  has  remarked  that  these  Eurypterids  were  at 
their  maximum  in  powers  of  variation  and  multiplication 
at  the  very  time  required  by  Dr.  Gaskell's  theory,  which 
has,  however,  not  been  widely  accepted. 

The  investigation  of  the  fossil  animals  with  backbones 
is  difficult,  as  it  is  comparatively  seldom  that  the  whole 
skeletons  are  found.  On  the  death  of  an  animal  its  bones 
are  usually  scattered,  and  many  important  extinct  creatures 
are  known  only  from  a  few  teeth,  or  the  lower  jaw,  or 
some  particularly  massive  bone.  The  reconstructions  of 
the  whole  animal  from  such  fragments  have  been  accom- 
panied by  many  mistakes.  The  great  French  naturalist, 
Cuvier,  held  that  a  particular  form  of  one  part  of  the  body 
was  always  associated  with  the  corresponding  form  in 
another.  "  Show  me  the  teeth,"  he  argued,  and  he  would 
deduce  from  it  the  characters  of  the  limbs  ;  and  from  the 
limbs  he  thought  it  always  possible  to  determine  the 
nature  of  the  backbone  and  thus  the  whole  skeleton. 
Conversely  he  held  that  as  a  footprint  gives  the  form  of 
the  foot,  he  could  determine  from  it  the  character  of  the 
limbs,  and  thus  work  back  to  the  nature  of  the  teeth  and 
the  shape  of  the  skull.  This  rule  proved  to  be  fairly  cor- 
rect so  long  as  it  was  applied  only  to  comparatively  recent 
fossils,  which  are  similar  in  all  their  chief  characters  to 
living  animals ;  but  the  application  of  this  principle  to  the 
early  mammals  and  reptiles  led  to  many  mistakes.  Thus 

240 


The  Interpretation  of  Fossils 

footprints  were  identified  as  those  of  birds,  although  they 
were  made  long  before  birds  appeared  upon  the  earth, 
and  were  due  to  reptiles  which  had  a  bird-like  foot. 
Similarly  teeth  were  assigned  to  mammals  which  have 
proved  to  be  those  of  reptiles  ;  and  Noctharctus,  which 
lived  in  the  Eocene  Period  in  North  America,  was  re- 
garded from  its  teeth  as  a  hoofed  quadruped,  whereas  it 
was  a  primitive  monkey. 

Cuvier's  principle  is  unreliable,  since  the  form  of  the 
body,  the  structure  of  the  skeleton,  and  the  nature  of  the 
teeth  are  dependent  on  the  habits  of  the  animal  and  not 
on  its  zoological  relationships.  Thus  when  animals  which 
belong  to  different  groups  have  adopted  the  same  mode  of 
life,  they  have  acquired  the  same  kinds  of  teeth,  the  same 
methods  of  defence,  and  their  bodies  have  been  gradually 
moulded  into  similar  forms.  There  is,  for  example,  a 
striking  external  resemblance  between  the  extinct  Ichthyo- 
saurus, which  is  a  reptile,  and  the  living  dolphins,  which 
are  mammals.  The  resemblances  are  due  to  the  teeth  and 
jaws  having  been  developed  into  the  form  most  effective 
for  the  capture  and  eating  of  fish,  and  the  shape  of  both 
animals  has  been  moulded  into  that  best  adapted  for  rapid 
passage  through  water.  The  shape  of  the  body  is  con- 
trolled by  the  same  mechanical  requirements  as  those 
which  determined  the  form  of  the  naval  locomotive  torpedo. 

Again,  the  Australian  mole  is  similar  in  habits  and  form 
to  the  British  mole,  although  they  have  both  been  derived 
from  ancestors  very  different  in  characters ;  and  members 
of  several  groups  of  animals  have  acquired  the  gait  of  the 
kangaroo,  and  the  power  of  flight  has  been  independently 
developed  by  various  reptiles  and  mammals  as  well  as  by 
birds. 

The  characteristic  feature  of  the  vertebrate  animals  is 
the  backbone,  which  consists  of  a  chain  of  bones  known 
as  "  vertebrae."  It  forms  the  main  support  of  the  body. 

241  *  Q 


The  Interpretation  of  Fossils 

The  backbone  usually  bears  two  groups  of  bones  arranged 
as  arches.  The  arms,  wings,  or  forelegs  are  attached  tc 
the  front  arch ;  the  legs,  or  the  back  legs  on  those  with 
more  than  one  pair,  are  supported  by  the  hinder  arch.  Al 
the  front  end  of  the  backbone  is  a  bony  case,  the  skull, 
which  supports  the  jaws  and  encloses  the  brain. 
The  Vertebrate  animals  include  the  following : 

Fish. 

Amphibians  (frogs,  newts,  etc.). 

Reptiles. 

Birds. 

Mammals. 

The  fish  are  the  most  primitive  class,  and  they  are  alsc 
the  oldest.  They  first  appear  in  the  Silurian  Period 
the  remains  of  ancient  sharks  and  of  armoured  jawless  fist 
(Ostracodermi)  of  this  Period  have  been  found  near  Ludlow 
in  Pennsylvania,  and  in  the  island  of  Oesel  in  the  Baltic 
The  most  primitive  known  fish  was  found,  however,  ir 
the  succeeding  system.  It  was  discovered  by  the  lat< 
distinguished  oculist,  Dr.  Marcus  Gunn,  in  the  Old  Rec 
Sandstone  flags  of  Caithness;  this  simple  fish,  Palczospon 
dylus  gunni  (Gr.  palaios,  ancient ;  spondulos,  a  joint  of  th< 
backbone),  was  two  inches  long;  it  had  no  known  fins 
the  skull  was  very  primitive,  with  a  ring  of  projecting 
processes  around  the  mouth,  and  it  had  no  jaws.  Behim 
the  skull  was  a  backbone  composed  of  rings ;  toward  thi 
tail  these  rings  bear  spines,  which  doubtless  supported  ; 
broad  tail  fin ;  there  is  no  trace  of  paired  limbs,  so  thes 
were  probably  soft,  finger-like  tentacles.  This  fish  ha 
been  regarded  as  a  fossil  lamprey.1 

The  oldest  known  fish  are  sharks,  of  which  have  beei 
found  the  long  spines  that  gave  the  necessary  firmnes 
to  the  fins  on  the  back.  Some  of  the  later  sharks  an< 
rays  are  better  known ;  their  hard  teeth,  some  of  whicl 
were  massive  plates  used  for  crushing  shellfish  and  grindin; 

242 


The  Interpretation  of  Fossils 

corals,  are  common  fossils.  The  allied  fish,  the  rays, 
which  include  the  common  skate,  have  yielded  some  of 
the  largest  known  fossil-fish,  while  teeth  are  known  which 
suggest  that  some  extinct  sharks  reached  the  length  of 
one  hundred  feet. 

The  most  remarkable  of  the  primitive  fish  were  the 
armoured  limbless,  jawless  fish  known  from  their  platy 
covering  case  as  the  Ostracodermi,  which  lived  only  in 
the  Silurian  and  Devonian  Periods.  Their  true  affinities 
are  still  uncertain,  and  for  long  it  was  doubtful  whether 
they  were  fish  ;  their  bony  plates  were  regarded  as  perhaps 
those  of  tortoises,  or  perhaps  even  belonging  to  an  animal 
related  to  the  scorpions  and  king  crabs.  These  animals 
had  no  paired  limbs  and  no  lower  jaw,  and  the  body  was 
sometimes  wholly  enclosed  by  plates  of  bone  with  the 
interspaces  filled  by  a  mosaic  of  bony  scales ;  but  in  others 
the  armour  was  reduced  to  scattered  bony  tubercles  and 
scales. 

Some  of  the  most  remarkable  of  these  animals  have 
been  discovered  in  the  Silurian  rocks  of  Lanarkshire ;  but 
the  best-known  occur  in  the  Old  Red  Sandstones.  In 
Cephalaspis,  the  head  was  protected  by  a  flat  shield,  and 
the  rest  of  the"*fiteck  was  covered  by  square  or  oblong  scales. 
Drepanaspis  (Gr.  drepane,  a  sickle;  aspis,  a  shield),  from  the 
Eifel,  a  district  in  Germany,  has  a  large  expanded  head, 
ten  inches  long,  which  is  protected  by  a  series  of  bony 
plates ;  the  tail  is  six  inches  long,  and  is  compressed  from 
side  to  side  and  covered  by  numerous  angular  scales. 
Pterichthys  (winged  fish),  which  will  always  be  associated 
with  the  memory  of  Hugh  Miller,  the  Scottish  geologist 
and  writer,  had  both  its  head  and  body  protected  by  a 
dozen  closely-fitting  bony  plates,  and  was  provided  with 
a  pair  of  armoured  paddles  and  a  scaly,  finned  tail. 

In  the  Devonian  times  these  jawless  fish  were  associated 
with  jaw-bearing  fish ;  in  these,  also,  the  body  was  pro- 

243 


The  Interpretation  of  Fossils 

tected  by  an  armour  of  bony  scales  or  plates,  while  the 
internal  bones  consisted  only,  or  mainly,  of  soft  cartilage. 
They  had  two  pairs  of  fins  in  addition  to  those  on  the 
middle  line  of  the  back  and  in  the  tail.  Most  of  them  had 
sharp  pointed  teeth,  though  some  were  toothless,  and 
others  had  the  jaws  covered  with  a  pavement  of  crushing 
plates. 

Such  fish  had,  therefore,  greater  powers  of  attack  than 
those  without  lower  jaws,  and  though  they  doubtless 
developed  later,  they  soon  displaced  their  feebler  pre- 
decessors. These  armoured  fish,  or  Ganoids  (Gr.  ganos, 
brightness;  eidos,  form),  were  the  most  powerful  of  all 
fishes,  from  soon  after  their  appearance  until  the  beginning 
of  the  Chalk  Period.  But  they  were  not  for  the  whole  of 
this  time  the  masters  of  the  Ocean,  for  their  supremacy 
was  lost  after  the  appearance  of  sea-living  reptiles ;  and 
thus  the  Devonian  or  Old  Red  Sandstone  is  called  the 
"  Age  of  Fish,"  as  they  were  then  abundant  and  were  the 
most  highly-developed  animals  alive  in  the  world. 

The  armoured  fish  were  supplanted  in  the  Chalk  Period 
by  the  ordinary  bony  fish  in  which  the  skeleton  consists 
of  numerous  internal  bones.  The  "  bony  fish "  include 
all  the  common  fish  of  the  present  day,  such  as  the  salmon, 
herring,  mackerel,  sole,  and  plaice.  They  are  descendants 
from  the  armoured  fish,  and  were  first  developed  in  the 
age  of  the  New  Red  Sandstone  (Trias).  A  still  undis- 
covered fish  must  then  have  had  its  bony  plates  reduced 
to  thin  overlapping  scales.  This  fish  was  the  ancestor  of 
two  distinct  lines  of  descendants;,  one  series  retained 
external  armour,  and  it  is  now  represented  by  Amia, 
which  still  lives  in  the  southern  rivers  of  the  United 
States  ;  the  other  line  of  descent,  by  the  further  reduction 
of  the  scales  and  the  development  of  the  internal  bones, 
and  especially  of  the  backbone,  gave  rise  to  some  such  bony 
fish  as  Lep.tolepis2  (Gr.  leptos,  delicate  ;  lepis,  a  scale).  The 

244 


The  Interpretation  of  Fossils 


A 


latter  fish  and  its  allies  formed  the  most  abundant  bony 
fish  through  the  Jurassic.  At  the  beginning  of  the  Period 
of  Chalk  (i.e.,  in  Lower  Cretaceous  times),  in  addition  to 
Leptolepis,  there  appeared  some  bony  fish,  such  as  Eury- 
pholis  (Gr.  eurus,  wide;  pholis,  a  scale3),  from  the  Adriatic 
area,  which  are  so  different  from  Leptolepis  that  they  are 
thought  to  have  descended  from  quite  a  different  Group 
of  armoured  fish ;  and  this  double  origin  of  the  bony  fish 
helped  their  rapid  increase  in  number  and  variety.  Before 
the  end  of  the  Chalk  Period  the  bony  fish  had  displaced 
the  armoured  fish  from  their  dominant  position ;  the 
armoured  fish  disappeared  from  the  sea,  and  only  a  few 
representatives  of  them  still  survive  in  lakes  and  rivers, 
such  as  the  sturgeon  of  eastern  Europe,  the  bony  pike, 
Polypterus  (Gr. polus,  many;  pteron,  fin)  of  the  Nile,  and 
the  bony  garfish,  Lepidosteus  (Gr.  lepis,  a  scale ;  osteon,  a 
bone),  of  Lake  Superior. 

The  most  striking  feature  in  the  life  of  the  Carboniferous 
Period  was  the  development  of  the  first  land-living  back- 
boned animal.  It  probably  spent  most  of  its  life  in  water, 
but,  being  amphibious,  it  and  its  allies  are  known  as  the 
"  amphibians."  The  young  always  live  in  water  and  have 
gills,  while  the  adults  live  on  land  and  breathe  by  lungs. 
The  best-known  representatives  of  this  class  to-day  are 
the  frogs  and  toads,  and  the  more  primitive  surviving 
forms  are  the  newts. 

The  most  important  feature  of  the  amphibians  is  the 
lung.  It  was  not,  however,  quite  a  new  structure,  for  the 
members  of  one  group  of  fishes  have  lungs  and  breathe 
air.  They  are  the  primitive  mudfish,  such  as  Protopterus 
(Gr.  protos,  first ;  pteron,  a  fin),  which  lives  in  the  mud  of 
brackish  water  lagoons  and  estuaries  of  equatorial  Africa, 
and  the  Barramunda,  or  Mary  River  Salmon,  the  great 
lung-fish  of  Queensland,  Ceratodus  (Gr.  keras,  a  horn  ; 
odous,  a  tooth). 

245 


The  Interpretation  of  Fossils 

Protopterus  (Gr.  protos,  first ;  pteron,  a  fin)  being  a  soft- 
bodied  animal  has  no  fossil  representatives;  but  fortunately 
Ceratodus  has  two  pairs  of  large  massive  teeth,  and  these 
are  well  preserved  as  fossils.  This  fish  was  known  to  have 
been  alive  in  the  earliest  part  of  the  Mesozoic ;  and  it  was 
preceded  in  Palaeozoic  times,  as  far  back  as  the  Devonian, 
by  lung-fish,  such  as  Dipterus,  which  was  in  many  ways 
allied  to  the  armoured  fish.  The  lung-fish  were  probably 
developed  from  some  armoured  ancestor  that  inhabited 
swamps  and  lagoons,  and  thus  had  at  times  to  live  through 
periods  when  the  water  had  dried  up  and  the  fish  survived 
by  hybernating  in  the  mud  on  the  floor. 

The  modern  air-breathing  lung-fish  have  lost  the  two- 
pointed  tail  present  in  most  fish.  Their  paired  fins  are 
long,  narrow,  and  cylindrical,  instead  of  being  flat  and 
built  up  of  many  parallel  bones  or  of  a  fan-shaped  group 
of  bones,  as  in  most  armoured  fish ;  and  as  the  lung-fish 
scriggled  about  on  the  soft  mud  their  rod-like  limbs  were 
more  useful  than  swimming-paddles.  Some  descendants 
of  the  lung-fish  further  developed  their  long  narrow  limbs 
into  the  walking  legs,  and  the  amphibians  had  legs  con- 
taining the  mammals'  arrangement  of  bones  in  and  ending 
with  five  toes ;  so  they  could  at  first  crawl  about,  and  then 
by  the  increase  in  the  strength  of  the  limbs  could  walk 
with  the  body  slightly  raised  off  the  ground. 

The  earliest  of  the  amphibians  had  several  resemblances 
to  the  armoured  fish  as  well  as  to  the  lung-fish ;  for  usually 
the  joints  of  the  backbone  were  but  partially  formed  of 
bone,  and  the  body  was  protected  by  an  armour  of  bony 
plates  and  scales,  which  are  absent  from  all  living 
amphibians.  The  teeth  also  resemble  those  of  fish  in 
both  succession  and  structure.  A  new  tooth  does  not 
grow  up  as  in  reptiles  below  an  old  tooth  and  push  it 
outward,  but  grows  between  two  old  ones  and  squeezes 
them  out. 

246 


FLYING  REPTILES  OF  THE  JURASSIC  PERIOD 

The  short-tailed  reptiles  at  the  top  and  bottom  are  species  of  Pterodactyles  (P.  crassirostris) 
which  were  as  large  as  rooks  :  the  long-tailed  forms,  one  of  which  is  climbing  up  the  tree,  is 
a  Rhamphorhynchus,  of  which  the  body  was  also  about  as  large  as  that  of  a  rook.  The 
small  Pterodactyle  on  the  trunk  to  the  right  of  the  picture  (P.  spectabilus)  was  about  the 
size  of  a  sparrow.  The  large  dragon-flies  of  the  same  period  are  also  shown. 


The  Interpretation  ot  Fossils 

The  amphibians  were  the  highest  forms  of  life  in  the 
Carboniferous  Period,  which  is  therefore  called  the  "  Age 
of  Amphibia."  They  soon  developed  to  great  size  and 
variety  of  form.  Some  of  them,  the  Labyrinthodonts, 
had  skulls  as  much  as  four  feet  long.  These  Labyrintho- 
donts owe  their  name  to  the  vertical  layers  in  the  teeth 
being  twisted  into  labyrinthine  patterns;  and  this  twist- 
ing of  the  tooth  substance  (dentine)  occurs  also  in  some 
Devonian  fishes,  as  in  Holoptychius  (all  folded :  Gr.  holos, 
whole ;  ptuchion,  folded,  in  folds  as  of  a  garment  or  hills). 
These  early  amphibians  belong  to  the  Group  of  the 
Stegocephalia  (roof-headed:  Gr.  stegos,  a  roof  or  covering) ; 
and  with  the  exception  of  a  few  genera,  they  are  all  four- 
limbed  and  have  a  long  tail,  and  thus  resemble  the  modern 
salamanders.  Some  were  as  large  as  modern  crocodiles, 
for  Mastodonsaurus  (a  round-toothed  saurian)  of  Wiirtem- 
berg  had  a  skull  four  feet  long  (PI.  XII.);  they  lived  from 
the  Carboniferous  to  the  Trias,  and  some  on  land  and 
some  in  fresh  water. 

The  Stegocephalia  (or  roof-headed  amphibians)  became 
extinct  at  the  end  of  the  Trias,  although  they  were  living 
in  all  parts  of  the  world,  including  South  Africa  and 
Australia;  and  during  the  next  Period,  the  Jurassic,  no 
amphibians  have  been  found.  The  class  reappears,  how- 
ever, in  the  Chalk  Period  with  some  long-tailed  newts 
allied  to  the  Salamanders,  and  a  large  skeleton  of  one  of 
these  creatures  which  lived  in  Switzerland  later  on  was 
figured  and  described  as  the  skeleton  of  a  man  and  claimed 
as  proof  of  the  Noah's  deluge ;  this  claim  was  made  by  the 
naturalist  Scheuchzer,  who  called  the  fossil  Homo  diluvii 
testis,  "the  man  a  witness  of  the  deluge";  but  it  is  now 
known  as  Cryptobranchus  scheuchzeri  (Scheuchzer's  "hidden- 
lunged  animal "). 

The  frogs  made  their  appearance  in  the  Eocene  beds, 
associated  with  the  great  lava-fields  of  western  India. 

247 


The  Interpretation  of  Fossils 

The  Carboniferous  Period  shows  that  the  land  when 
well  watered  was  already  densely  covered  with  vegetation  ; 
this  gave  the  land  animals,  as  soon  as  they  were  developed, 
ample  supplies  of  vegetable  food.  Consequently  the 
amphibians  increased  rapidly  in  number,  and  spread  from 
Europe  and  Asia  to  America,  South  Africa,  and  Australia. 

The  most  specialized  of  the  Carboniferous  amphibians 
had  gradually  lost  the  fish-like  characters  present  in  the 
more  primitive  members  of  the  class;  thus  the  joints  of 
the  backbone  had  been  fully  converted  into  bone,  and 
the  armour  was  reduced  in  extent,  and  was  probably 
absent  from  the  great  Mastodonsaurus  (the  round-toothed 
saurian) ;  and  the  animals  were  terrestrial  for  most  of 
their  life,  and  they  left  their  big  five-toed  or  hand-like 
footprints  on  the  wet  sands  beside  the  New  Red  Sand- 
stone lakes  and  lagoons.  In  most  respects  the  higher 
amphibians  approach  the  reptiles.  Some  were  even 
serpent-like  in  form,  as  they  were  legless  and  had  very 
long  snake-like  bodies,  such  as  the  Dolichosoma  (long- 
bodied  :  Gr.  dolichos,  long ;  soma,  a  body),  which  was  de- 
scribed by  Huxley  from  the  Coal  Measures  of  Ireland ; 
and  this  type  is  still  represented  by  the  Ccecilians  which 
live  in  tropical  India,  Africa,  and  South  America. 

Ultimately,  in  the  continental  lands  of  the  Old  World, 
the  amphibians  developed  into  reptiles. 

1  According  to  recent  nomenclature,  the  jawless  fish  are  not 
included  in  the  class  Pisces,  the  members  of  which  are  regarded  as 
"  the  true  fishes."  Lampreys  and  their  extinct  allies  form  a  distinct 
class,  Agnatha  (a,  not ;  gnathos,  jaw).  The  word  "  fish  "  in  its  ordinary 
meaning  includes  both. 

8  It  is  found  in  the  Upper  Lias  of  Europe,  and  spread  so  rapidly 
through  the  world  that  it  soon  reached  Australia,  where  its  remains 
are  found  in  the  Wianamatta  beds  of  New  South  Wales. 

3  Gr.  phtflis,  a  scale  of  a  reptile  ;  lepis,  a  scale  of  a  fish. 


248 


CHAPTER  XVIII 
THE  ANCIENT  REPTILES  AND  THE  ORIGIN  OF  BIRDS 

THE  reptiles  are  so  similar  in  form  and  habits  to  the 
amphibians  that  it  is  sometimes  doubtful  in  which  class 
the  animals  should  be  included.  The  amphibians  and 
reptiles  agree  in  being  cold-blooded,  and  in  usually  having 
four  walking  legs,  though  legless  creeping  animals  occur 
in  both  groups.  The  two  essential  differences  between 
the  classes  are  that  the  skull  of  amphibians  is  attached 
to  the  backbone  by  two  joint-surfaces,  whereas  reptiles 
have  only  one  joint-surface,  and  that  the  reptiles  breathe 
throughout  life  by  lungs  and  never  have  gills.  The  more 
striking  differences  are  in  external  appearance :  the  reptiles 
are  longer  than  amphibians,  and  far  more  varied  in  form 
and  habit.  The  living  reptiles  include  snakes,  crocodiles, 
and  lizards,  while  the  extinct  saurians  lived  on  land  or  in 
the  sea  or  flew  in  the  air  like  bats,  and  some  were  so 
mammal-like  that  their  fossil  remains  were  once  identified 
as  those  of  mammals. 

The  oldest  known  reptile,  Proterosaurus  (Gr.  proteros, 
earlier,  older),  lived  in  the  Lower  Permian ;  it  was  a 
lizard-like  animal,  and  grew  to  the  length  of  about  five 
feet.  By  Upper  Permian  times  the  amphibians  had  be- 
come abundant  in  England,  at  Elgin  in  Scotland,  in 
Germany,  North  America,  and  South  Africa.  They  were 
the  dominant  animals  throughout  the  Mesozoic,  which  is 
therefore  known  as  the  "Era  of  Reptiles." 

The  most  primitive  reptiles  are  known  as  the  Rhyncho- 

249 


Ancient  Reptiles  and  Origin  of  Birds 

cephalia  (beak -headed:  Gr.  rhynchos,  a  beak  or  snout; 
kephale,  a  head),  of  which  there  is  one  living  representa- 
tive, the  Sphenodon  (Gr.  sphen,  a  wedge),  of  New  Zealand. 
This  animal  is  remarkable  for  having  a  third  eye  on  the 
middle  of  the  forehead.  As  there  is  a  large  pit  in  a  corre- 
sponding position  in  many  extinct  reptiles,  they  doubtless 
had  a  well-developed  third  eye,  so  that,  as  they  floated 
with  the  head  on  the  surface  of  the  water,  they  could 
see  either  foes  or  food  above  them. 

This  primitive  group  gave  rise  to  a  varied  order  of 
scale-coloured  reptiles,  including  the  lizards,  snakes,  and 
their  extinct  allies,  like  the  gigantic  Mosasaurus,  which 
was  forty  feet  long,  and  swam  in  the  seas  at  the  end  of 
the  Chalk  Period.  The  largest  of  the  lizards,  Megalania 
(Gr.,  megas,  big ;  and  Lat.,  lanius,  a  butcher),  was  about 
thirty  feet  long,  and  lived  in  quite  recent  times  in  Queens- 
land and  in  the  swamps  around  Lake  Eyre  in  central 
Australia.  Of  the  snakes  comparatively  few  have  been 
found  as  fossils,  though,  as  the  snakes  are  mainly  tropical, 
more  will  be  found  when  the  Kainozoic  beds  of  the  tropics 
have  been  more  thoroughly  explored.  But  some  great 
pythons  are  known  from  the  Eocene  beds  of  the  Isle  of 
Sheppey,  which  may  have  been  twenty  feet  long. 

The  reptiles  multiplied  apace,  and  soon  adapted  them- 
selves to  many  different  modes  of  life.  They  were  affected 
by  the  same  mechanical  conditions  as  those  which  subse- 
quently controlled  the  development  of  the  mammals. 
The  reptiles  had  the  drawbacks  of  inferior  mechanism 
and  intelligence,  but  they  acquired  similar  general  forms. 
Hence  we  find  in  the  Era  of  Reptiles  creatures  with  very 
similar  forms  to  those  that  lived  in  the  Era  of  Mammals. 
Thus  the  skull  of  the  giant  Iguanodon  (toothed — like  the 
Iguana,  a  South  African  lizard)  has  a  remarkable  general 
resemblance  to  that  of  the  horse,  but  while  erect  the 
animal  had  the  tripod  attitude  of  the  kangaroo. 

250 


Ancient  Reptiles  and  Origin  of  Birds 

The  Crocodiles  form  one  of  the  most  ancient  groups  of 
reptiles  of  which  representatives  still  survive.  They  are 
no  doubt  direct  descendants  of  the  beak-headed  reptiles 
(Rhynchocephalia),  for  the  crocodiles  that  lived  in  the  New 
Red  Sandstone  were  very  closely  allied  to  the  reptiles  of 
that  primitive  order.  These  earliest  crocodiles  agree  with 
their  beak-headed  predecessors  in  the  nature  of  the  armour- 
plate  which  protects  the  under  side  of  the  body,  in  the 
presence  of  abdominal  ribs,  and  many  details  in  the 
anatomy  of  the  skull. 

The  oldest  remains  of  crocodiles  in  the  British  Isles  are 
known  from  casts  of  their  bones  wrhich  have  been  found  in 
the  New  Red  Sandstones  of  Elgin.  This  rock  was  re- 
garded as  belonging  to  the  Old  Red  Sandstones  until 
Prof.  Huxley  examined  some  cavities  in  the  rock,  and 
discovered  from  casts  of  them  that  they  were  spaces  left 
by  the  removal  of  the  bones  of  primitive  crocodiles.  In  a 
famous  paper  he  showed  that  these  beds  could  not  be  older 
than  the  New  Red  Sandstone,  and  pointed  out  the  lines 
of  evolution  of  the  crocodiles. 

Crocodiles  still  more  similar  to  those  of  the  present  day 
came  into  existence  shortly  after  the  end  of  the  New  Red 
Sandstone,  and  as  some  of  them  were  twenty  feet  long, 
they  were  nearly  as  large  as  those  of  to-day.  The  Group, 
which  includes  the  recent  alligators  and  crocodiles,  did 
not  appear  until  the  beginning  of  the  Chalk  Period ;  they 
were  abundant  later  in  eastern  Kent  in  the  estuary  wherein 
was  deposited  the  London  Clay.  The  largest  extinct 
crocodiles  were  allies  of  the  Indian  Gavials,  which 
lived  in  Pliocene  times  in  the  Siwalik  Hills  of  India ; 
they  are  estimated  to  have  been  almost  fifty  feet  long, 
or  more  than  twice  the  length  of  the  largest  existing 
crocodiles. 

The  crocodiles  are  not  only  of  interest  as  a  very  ancient 
Group,  but   as  the  ancestors  of  the   reptiles  classed  as 

251 


Ancient  Reptiles  and  Origin  of  Birds 

Dinosaurs,  from  which  the  birds  have  been  in  turn 
derived. 

The  oldest  of  the  reptiles  belong  to  the  Group  known 
as  the  Theromorpha,  which  includes  some  of  the  most 
remarkable  of  them  all ;  the  name  of  the  order  is  derived 
from  the  Greek  words,  therion,  a  beast ;  morphe,  form,  as 
they  have  the  form  of  mammals.  They  are  an  ancient 
and  extinct  Group,  and  had  a  comparatively  short  life. 
They  began  in  the  Lower  Permian,  and  disappeared  at 
the  end  of  the  next  Period,  the  New  Red  Sandstone.  The 
earliest  of  the  Theromorphs  was  descended  from  the  am- 
phibian Mastodonsaurus  or  from  some  near  ally,  which 
adopted  an  entirely  land  life;  it  lost  its  gills,  breathed 
wholly  by  lungs,  and  gradually  changed  its  crawl  for  a 
walk  as  its  five-toed  limbs  were  strengthened,  so  that 
it  could  at  last  walk  with  the  body  raised  off  the  ground. 
The  under  side  of  the  body  being  sheltered  and  not  ex- 
posed to  rough  contact  with  the  ground  had  no  abdominal 
ribs. 

The  hind  limbs  in  the  Theromorpha  were  attached  to  an 
arch  of  bones,  containing,  as  in  other  vertebrates,  three 
bones  on  each  side  ;  but,  unlike  any  other  reptiles,  the 
three  bones  on  each  side  have  grown  together  in  one  con- 
tinuous bone.  This  arrangement  is  found  elsewhere  only 
in  the  mammals,  where  the  hip-bone  on  each  side  of  the 
animal  consists  of  one  bone,  due  to  the  fusion  of  three 
originally  separate  bones.  The  Theromorphs  also  resemble 
the  mammals  in  the  nature  of  their  teeth.  In  ordinary 
reptiles  the  teeth  may  be  numerous,  large,  and  important ; 
but  they  are  either  all  alike,  as  in  alligators,  or  they  are 
all  on  the  same  pattern,  though  some  may  be  longer  than 
the  rest,  as  in  the  crocodile.  In  mammals,  except  in  those 
that,  like  porpoises,  live  in  the  sea  and  feed  on  fish,  the 
teeth  are  varied  in  shape.  There  are  three  chief  kinds : 
the  cutting  teeth,  or  incisors,  in  front ;  behind  these  on 

252 


Ancient  Reptiles  and  Origin  of  Birds 

each  side  of  each  jaw  is  usually  a  large  conical  tooth,  the 
dog-tooth,  or  canine,  which  is  used  for  piercing  or  holding 
prey ;  and  behind  are  a  series  of  teeth  which  are  usually 
flat-topped  or  ridged,  and  are  used  for  grinding  and  masti- 
cating the  food.  These  are  the  molars  (from  Lat.  mola,  a 
mill).  In  the  carnivorous  forms  the  molar  teeth  may  be 
conical,  though  they  are  then  usually  flattened,  and  have 
sharp  cutting  edges. 

Thus  comparison  of  the  jaw  of  an  alligator  with  that 
of  a  dog  shows  the  contrast  between  the  succession  of 
similar  teeth  in  the  reptile  and  the  varied  teeth  of  the 
mammals.  The  Theromorphs  have,  however,  teeth  modi- 
fied to  act  as  incisors,  canines,  and  molars ;  as  in 
Cynognathus.  Hence  when  these  teeth  were  first  dis- 
covered they  were  naturally  regarded  as  belonging 
to  mammals.  But  they  represent  a  case  of  a  similar 
structure  having  been  independently  developed  to  serve 
similar  needs. 

The  earliest  of  the  Theromorphs  fed  on  vegetable  food, 
and  lived  during  the  time  of  the  Permian  and  New  Red 
Sandstone  in  a  land  which  extended  north  and  south, 
in  the  Old  World  from  northern  Europe  to  South  Africa ; 
for  they  have  been  found  from  Russia  and  Elgin  in  Scotland 
to  Cape  Colony,  while  distinct  though  allied  animals  lived 
in  Texas. 

The  most  remarkable  of  these  plant-eating  Permian 
forms  is  the  Pariasaurus,  which  had  a  flat,  large  body  up 
to  eight  feet  long,  supported  on  four  squat  bent  legs,  like 
those  of  a  pug  dog  or  dachshund.  It  had  small  teeth. 
An  allied  kind,  Elginia,  from  the  New  Red  Sandstone 
of  Elgin,  had  the  head  defended  by  several  pairs  of  horns, 
which  grew  upward  from  the  skull.  These  two  reptiles 
had  small  teeth  ;  but  the  development  of  herds  of  these 
herbivorous  animals  was  soon  followed  by  the  appearance 
of  others  which  preyed  upon  them ;  these  were  the 

253 


Ancient  Reptiles  and  Origin  of  Birds 

carnivorous  Theromorphs,  in  which  the  teeth  were  all 
pointed,  and  the  canines  were  large  tusks,  like  those  of  the 
tiger,  and  could  easily  pierce  the  brain  of  any  soft-skulled 
animal,  or  tear  across  the  throat  The  Lycosaurus  (lukos, 
a  wolf)  and  Cynognathus  (dog-jawed)  of  South  Africa  were 
two  of  the  best  known  of  these  carnivorous  forms  ;  Cyno- 
gnathus had  a  skull  sixteen  inches  long  with  large  canines, 
and  in  some  respects  was  remarkably  similar  to  the  bears 
of  our  time. 

The  development  of  such  powerful  carnivores  led  in 
turn  to  strange  defences  on  the  part  of  the  vegetarian 
Theromorphs.  The  Permian  Dimetrodon  of  Texas  (which 
according  to  some  authorities  is  one  of  the  beaked  reptiles, 
or  Rhynchocephalia)  grew  a  series  of  spines  upward  from 
its  backbone,  so  that  they  projected  along  the  middle  line 
of  the  back,  and  thus  defended  the  animal  from  attack 
from  above. 

Another  line  of  development  gave  rise  to  a  group  of 
beaked  reptiles  with  no  teeth,  or  only  a  pair  of  front  tusks. 
These  are  the  Dicynodons,  which  have  been  found  in 
South  Africa,  India,  Russia,  and  Scotland. 

The  most  important  part  which  the  Theromorphs  played 
in  the  history  of  the  world  was  as  the  ancestors  of  the 
mammals.  One  group,  the  Tritylodons,  had  in  their 
skulls  several  of  the  characteristics  of  mammals ;  the  most 
striking  resemblance  was  in  the  molar  teeth,  which  were 
provided  with  two  or  three  rows  of  small  tubercles  ;  these 
teeth  were  at  first  believed  to  belong  to  mammals.  The 
Dicynodons,  owing  to  their  beaks  and  the  structure  of  the 
shoulder-blade,  agree  with  the  most  primitive  of  the 
mammals,  the  Duck-Billed  Platypus  of  Australia ;  and 
t  appears  probable  that  one  group  of  Theromorphs  gave 
rise  to  the  egg-laying  mammals,  the  Monotremes  (single 
orifice) ;  while  the  Tritylodon  (three-knobbed  teeth)  gave 
rise  to  small  mammals  which  have  teeth  provided  with 

254 


o     2  w> 

I 


Pi        J3 

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Ancient  Reptiles  and  Origin  of  Birds 

many  small  tubercles,  and  suckle  their  young  and  never 
lay  eggs. 

Though  the  mammals  were  thus  initiated  in  the  Trias, 
their  progress  was  slow,  and  the  Dinosaurs,  descendants 
of  the  Crocodiles,  soon  acquired  supremacy  on  land,  in 
sea,  and  in  air. 

Dinosaurs1  are  long-bodied  reptiles,  with  long  neck  and 
tail,  and  the  hind-legs  are  usually  much  larger  than  the 
fore-limbs;  the  animal  was  often  kangaroo-like  in  gait, 
and  had  no  third  eye  and  no  abdominal  ribs.  The  bones 
of  the  hip  girdle  were  similar  in  several  respects  to  those 
of  birds.  Some  Dinosaurs  were  of  colossal  size,  and  were 
among  the  largest  animals  that  have  ever  lived,  though 
none  of  them  was  larger  than  the  biggest  existing  whale, 
and  apparently  none  of  those  that  lived  on  land  was  more 
massive  than  the  African  elephant.  The  head,  however, 
was  often  small,  and  the  brain  still  smaller  in  proportion. 
Thus  the  Dinosaur  Triceratops  (three-horned :  Gr.  opsis, 
appearance)  had  in  proportion  to  the  size  of  the  skull 
a  smaller  brain  than  any  other  backboned  animal  that 
has  lived  on  land. 

The  oldest  and  most  primitive  Dinosaurs  were  car- 
nivorous ;  and  as  they  are  first  known  from  the  New  Red 
Sandstones  of  Europe,  Africa,  North  America,  and  Asia, 
they  appear  to  have  spread  rapidly  through  all  the  lands 
of  the  world  that  were  then  connected.  The  vegetarian 
reptiles  and  amphibians  afforded  them  ample  food.  The 
carnivorous  reptiles  never  reached  so  large  a  size  as  those 
that  fed  on  plants.  Many  of  the  flesh-eating  Dinosaurs 
were  small  and  active.  Thus  Campsognathus  (bent-jaw), 
which  is  found  in  the  Lithographic  Stone  of  Bavaria,  was 
only  as  large  as  a  cat ;  it  had  a  very  bird-like  skull,  and 
must  have  been  very  agile ;  and  Hallopus,  which  lived  at 
about  the  same  time  in  Colorado,  probably  leapt  like 
a  kangaroo.  But  some  of  the  carnivorous  Dinosaurs 

255 


Ancient  Reptiles  and  Origin  of  Birds 

which  lived  in  the  beginning  of  the  Chalk  Period  and 
slightly  earlier,  were  of  considerable  size ;  the  biggest  of 
them  was  the  giant  Megalosaurus  of  the  Weald  of  Kent ; 
it  walked  on  its  two  hind-legs,  and  balanced  itself  by  its 
long  heavy  tail. 

The  remaining  divisions  of  Dinosaurs  were  herbivorous, 
a  fact  clearly  shown  by  the  nature  of  their  teeth  ;  some  of 
them  walked  on  the  hind-legs,  such  as  the  giant  Iguanodon 
which  lived  in  south-eastern  England  and  in  Belgium 
during  the  early  part  of  the  Chalk  Period  and  the  end 
of  the  preceding  or  Oolitic  Period.  It  was  first  known 
from  some  bones  found  near  Maidstone;  a  number  of  com- 
plete skeletons  have  been  obtained  from  a  fissure  at  Bernis- 
sart  in  Belgium,  and  are  now  in  the  Museum  in  Brussels. 
The  animal  (PI.  XIV.)  had  a  massive  elongated  skull, 
which  in  some  respects  was  like  that  of  a  horse.  It  used 
its  front  limbs  as  paws  ;  and  guided  its  food  to  its  mouth 
with  its  paws,  each  of  which  had  four  fingers  and  a  thumb 
that  was  reduced  to  a  spur-like  claw;  the  little  finger 
was  much  feebler  than  the  others.  Some  of  the  largest 
Iguanodons  must  have  been  over  thirty  feet  long,  and 
stood  fourteen  feet  high.  It  doubtless  walked  like  a 
kangaroo  on  its  hind-legs,  which  were  three-toed,  and 
balanced  itself  by  a  heavy  tail ;  but  it  probably  had  not 
the  jumping  powers  of  the  kangaroos.  The  legs  and  the 
arrangement  of  the  bones  to  which  they  were  attached 
were  bird-like.  Some  of  the  Dinosaurs  were  quadrupeds  ; 
but  according  to  Dr.  Dollo  they  regained  that  habit,  as 
their  ancestors  went  on  two  legs.  Some  of  these  four- 
legged  Dinosaurs  were  defended  by  long  rows  of  spines. 
Thus  Stegosaurus  (Gr.  stegos,  a  roof,  and  sauros,  a  lizard), 
which  was  thirty  feet  long  and  lived  during  the  Upper 
Jurassic  in  Colorado,  had  two  rows  of  massive  spines,  with 
twelve  in  each  row,  along  the  middle  of  the  back,  and  four 
pairs  of  spines  along  the  tail.  This  beast  needed  armour, 

256 


Ancient  Reptiles  and  Origin  of  Birds 

for  its  brain  was  smaller  in  proportion  to  the  size  of  the 
body  than  in  any  other  known  backboned  animal  that 
lived  on  land.  A  somewhat  similar  defence  was  adopted 
by  Triceratops,  which  was  as  large  as  a  rhinoceros,  and  its 
skull,  five  feet  long,  was  the  largest  in  any  known  land 
animal.  Projecting  upward  from  the  head  were  three 
short,  strong  horns,  and  any  animal  struck  by  the  upjerked 
head  of  Triceratops  must  have  been  seriously,  if  not  fatally, 
injured ;  but  in  proportion  to  the  size  of  its  skull  it  had 
the  smallest  known  brain  of  any  quadruped.  Another 
giant  was  the  Ceratosaurus,  which  was  fifteen  feet  long 
and  had  a  large  single  horn,  rising  from  the  front  of  the 
head  like  that  of  a  rhinoceros.  The  animal  was  also 
armed  with  two  upward  pointing  horns  on  the  top  of  the 
head,  and  it  probably  had  a  series  of  horny  spines  along 
the  crest  at  the  back  of  the  head  (PL  XV.). 

Another  series  of  four-legged  Dinosaurs  included  some 
of  the  largest  of  known  animals,  and  perhaps  the  largest 
that  ever  walked  on  land. 

Of  these  the  two  best-known  animals  are  the  Bronto- 
saurus  and  Diplodocus.  The  earliest  known  of  this  group 
is  Cetiosaurus,  which  was  first  discovered  in  the  Oolitic 
rocks  of  Oxfordshire,  and  though  only  part  of  its  skeleton 
is  known,  the  animal  must  have  been  at  least  forty  feet 
long.  The  two  best-known  animals  of  this  group  are  from 
the  Rocky  Mountains,  where  they  lived  in  Upper  Jurassic 
times.  Brontosaurus  was  fifty  feet  long,  while  Diplodocus 
was  eighty  feet. 

An  impression  of  the  size  of  the  latter  monster  can  be 
obtained  by  inspection  of  the  cast  of  the  skeleton  in  the 
Natural  History  Museum  in  London.  The  fossil  was 
excavated  by  Prof.  Holland  in  an  expedition  equipped  by 
Mr.  Andrew  Carnegie. 

Diplodocus  was  first  discovered  by  the  late  Prof.  O.  C. 
Marsh  in  1884,  and  the  hinder  part  of  a  large  skeleton 

257  R 


Ancient  Reptiles  and  Origin  of  Birds 

was  found  by  Prof.  Osborn,  of  the  Natural  History  Museum 
of  New  York,  in  1887.  This  specimen  had  fortunately 
been  exposed  by  a  gully,  which  had  cut  across  the  tail ; 
the  excavation  of  the  rest  occupied  several  months.  The 
size  and  general  construction  of  the  animal  can  be  best 
appreciated  from  the  specimen  subsequently  discovered 
by  Mr.  Carnegie's  expedition.  Its  length  was  seventy 
feet,  so  it  was  larger  than  an  elephant,  and  the  body  is 
estimated  to  have  weighed  twenty  tons.  The  legs  are 
comparatively  slim,  and  are  placed  near  the  middle  of  the 
body,  and  it  appears,  on  a  first  inspection,  almost  impos- 
sible that  they  could  have  supported  and  moved  so  great 
a  mass.  The  head,  it  is  true,  was  very  small,  and  most 
of  the  great  length  of  the  animal  was  in  its  neck  and  tail ; 
while  careful  study  of  the  bones  shows  that  they  were 
lightly  built. 

"  The  dominating  principle  in  construction  of  the  back- 
bone," says  Prof.  Osborn,  "  is  maximum  strength  with 
minimum  weight,"  and  he  refers  to  "the  ingenuity  of 
sculpture  by  which  this  is  brought  about,  every  single 
vertebra  differing  from  its  fellow." 

The  name  "  Diplodocus,"  means  double-raftered,  re- 
ferring to  the  large  rafter-like  outgrowths  from  the  joints 
of  the  backbone.  Prof.  Osborn  describes  the  skeleton 
of  Diplodocus  as  "a  marvel  of  construction.  It  is  a 
mechanical  triumph  of  great  size,  lightness,  and  strength." 

The  great  size  of  the  animal  is  not  only  explained  by 
the  lightness  of  the  bones,  but  by  the  abundant  evidence 
that  it  was  amphibious  in  habit.  This  was  recognized 
from  Marsh's  original  description  of  the  skull,  which 
showed  that  the  nostrils  opened  on  the  top  of  the  head,  as 
in  aquatic  animals.  Moreover,  the  tail  is  flattened  to  act 
as  a  propeller.  The  front  part  of  the  tail  was  broad,  and 
provided  with  very  powerful  muscles,  while  the  hinder  part 
was  compressed  from  side  to  side,  and  its  efficiency  as  a 

258 


Ancient  Reptiles  and  Origin  of  Birds 

swimming  organ  was  increased,  according  to  Prof.  Osborn, 
by  a  vertical  fin.  If  it  laid  eggs  it  must  have  crawled 
ashore  during  the  breeding  season ;  but  even  then  it  prob- 
ably spent  most  of  its  time  in  swamps,  so  that  much  of 
its  weight  was  supported  by  the  water.  In  all  likelihood 
it  fed  on  large  water-plants,  that  it  gathered  with  the 
claws  on  its  light  fore-limbs,  and  as  it  had  no  grinding 
teeth  it  possibly  swallowed  its  food  whole. 

The  remains  of  a  Dinosaur,  Gigantosaurus,  have  been 
discovered  in  German  East  Africa,  which,  judging  from 
the  size  of  some  bones,  was  much  larger  than  Diplodocus 
(PL  XVI.). 

While  various  land  reptiles  had  anticipated  the  forms 
of  land  mammals,  the  marine  Dinosaurs  had  acquired 
forms  similar  to  those  of  the  aquatic  mammals.  Thus  the 
Ichthyosaurs  (Gr.  ichthus,  a  fish)  were  strikingly  like  the 
porpoises ;  they  had  a  rounded  smooth  body,  ending  in 
front  in  a  pointed  head  and  long,  thin  snout,  so  that  it 
cleaved  its  way  readily  through  the  water ;  and,  like  the 
dolphins,  its  jaws  had  rows  of  many  pointed  teeth,  which 
would  at  once  pierce  and  hold  the  slippery  agile  fish  on 
which  it  preyed.  As  it  had  no  means  of  extracting  the 
bones  from  the  fish,  it  swallowed  them  whole,  and  so  had 
no  use  for  molars.  The  Ichthyosaur  was  well  provided 
with  teeth,  for  some  of  them  had  four  hundred ;  and  the 
teeth  show  traces  of  the  labyrinthine  folding  of  the  Laby- 
rinthodonts,  another  indication  of  their  descent  from  the 
Amphibia. 

There  can  be  no  doubt  that  the  Ichthyosaurs  were 
descended  from  a  terrestrial  quadruped,  for  the  earliest  of 
them,  which  lived  in  the  time  of  the  New  Red  Sandstone, 
had  thinner  and  less  efficient  paddles  than  their  successors 
in  the  Lias,  when  the  Ichthyosaurs  were  most  numerous 
and  powerful.  Their  adoption  of  a  marine  life  led  to  many 
changes  beside  their  fish-like  form;  for  the  tail,  by  the 

259 


Ancient  Reptiles  and  Origin  of  Birds 

development  of  some  sheets  of  skin,  acquired  a  fin,  and 
back  fins,  like  those  on  the  dolphins,  grew  along  the  middle 
line  of  the  back. 

As  the  Ichthyosaur  had  to  chase  its  prey  into  deep 
water,  where  the  light  was  dim,  it  required  large  eyes  so 
as  to  allow  the  entrance  of  the  maximum  of  light.  The 
eye  was  protected  by  a  ring  of  fifteen  to  nineteen  wedge- 
shaped  bony  plates,  for  the  pressure  in  deep  water  would 
have  injured  large,  soft,  unprotected  eyes;  as  these  plates 
were  pressed  inward,  they  fitted  together,  and  formed  a 
dome  which  protected  the  soft  internal  structures  of  the 
eye.  The  body  was  protected  from  water  pressure  by  a 
series  of  strong  ribs  developed  across  the  breast  and  belly. 
Another  modification  was  the  change  from  laying  eggs  to 
bringing  forth  the  young  alive.  The  Ichthyosaurs  could 
not  lay  eggs  by  the  million,  like  the  spawn  of  fish,  so  that 
the  individual  young  had  to  be  better  cared  for ;  and  as 
the  reptile  was  no  longer  able  to  go  on  land  to  lay  its  eggs 
it  brought  forth  its  young  able  to  swim  at  once,  and  soon 
lead  an  independent  existence. 

The  Plesiosaurs  (Gr.  plesios,  resembling,  or  allied  to)  were 
sea-living  reptiles,  but,  unlike  the  Ichthyosaurs,  they  were 
amphibious.  In  form  they  somewhat  resembled  the  sea- 
lion,  for  they  had  long  necks  and  four  short  limbs  on 
which  they  could  waddle  about  on  shore.  The  Plesiosaurs, 
however,  were  probably  more  awkward  on  land  than  the 
sea-lions,  and  spent  less  of  their  time  ashore,  though  they 
probably  went  there  to  lay  their  eggs.  They  had  a  small 
pointed  head  with  sharp,  conical  teeth,  like  those  of  the 
crocodile ;  the  neck  was  long  and  narrow,  like  that  of  a 
great  swan,  and  so  also  was  the  tail ;  the  body  was  small 
and  egg-shaped.  The  total  length  was  as  much  as  sixteen 
feet  (PL  XVI.). 

The  Plesiosaur  had  a  well-developed  third  eye,  and  it 
probably  secured  some  of  its  food  by  suddenly  snapping, 

260 


THE  PLESIOSAURUS,  A  MARINE  REPTILE 

It  lived  in  Jurassic  and  Cretaceous  times.     It  was  15  feet  long.     The  fore  limbs  were 
altered  into  swimming  paddles. 


Ancient  Reptiles  and  Origin  of   Birds 

with  a  swift  movement  of  its  long  neck,  the  fish  that  passed 
above  it.  Some  of  the  last  of  the  Plesiosaurs  may  have 
fed  on  birds  and  captured  those  that  passed  over  their 
heads  as  they  lay  hidden  just  below  the  surface  of  the  sea. 
The  third  eye  was,  however,  well  developed  in  the  earlier 
Plesiosaurs,  even  in  those  of  the  Lower  Lias,  when  there 
were  certainly  no  birds ;  and  though  there  were  a  few 
flying  reptiles,  their  clawed  feet  and  hands  show  that  they 
mainly  lived  on  land  and  in  forests ;  those  found  in  marine 
deposits  were  probably  blown  to  sea.  Hence  the  third 
eye  cannot  have  been  originally  developed  for  the  capture 
of  flying  animals.  The  Plesiosaurs,  though  most  likely 
less  able  swimmers  than  the  Ichthyosaurs,  probably  some- 
times visited  fairly  deep  water,  for  the  soft  structures  in 
the  abdomen  were  protected  by  front  ribs,  and  the  eye  by 
a  ring  of  bony  plates. 

The  group  of  reptiles,  including  the  turtles  and  the 
tortoises,  is  also  one  of  great  antiquity,  as  it  first  appeared 
in  the  New  Red  Sandstone,  and  has  remained  remarkably 
unchanged  in  form  ever  since.  The  tortoises  are  strikingly 
unlike  the  other  reptiles ;  their  horny  beak  is  bird-like, 
and  their  shell  of  closely-fitting  external  armour  resembles 
that  of  the  Armadilloes.  The  land  tortoises,  however,  are 
the  most  altered  members  of  the  order,  and  only  appeared 
in  the  first  age  of  the  Era  of  Mammals.  The  most 
primitive  turtles  lived  in  the  New  Red  Sandstone,  and 
had  a  skull  which  suggests  their  descent  from  the  Thero- 
morphs  (p.  252),  as  it  had  significant  resemblances  to  that 
of  the  Dicynodon  (double  dog-toothed:  Gr.  dis,  double; 
kuon,  dog). 

The  greatest  of  the  tortoises,  as  well  as  the  greatest  of  the 
crocodiles,  lived  in  India  in  Pliocene  times ;  for  the  great 
Testudo  atlas  of  the  Siwalik  Hills  had  a  shell  six  feet  long. 

The  ancestor  of  the  turtles  was  probably  a  sluggish 
beast  who,  to  save  a  frequent  race  for  life,  sought  shelter 

261 


Ancient  Reptiles  and  Origin  of  Birds 

in  swamps,  where  its  external  armour  increased  in  size, 
and  the  plates  became  dovetailed  into  a  firm  complete 
case.  Some  of  them,  thus  protected,  wandered  forth  into 
the  rivers,  and  found  a  more  varied  food;  they  travelled 
down  the  rivers  and  reached  the  sea  in  Upper  Jurassic 
times.  There  they  underwent  important  changes  in 
structure,  as  they  lived  more  by  swimming  than  crawling 
about  on  mud  banks  or  river  bottoms.  Their  river-living 
ancestors,  the  mud-turtles,  had  clawed  feet ;  but  the  sea- 
turtles  had  longer  unclawed  toes,  which  were  doubtless 
connected  by  a  web,  so  that  their  feet  acted  as  swimming- 
paddles.  While  the  armour  on  the  belly  remained  as  a 
firm  plate  which  gave  the  necessary  rigidity,  the  bones  of 
the  back,  as  that  part  was  no  longer  most  exposed  to 
attack,  became  soft  and  leathery  and  gave  the  animal  the 
advantage  of  greater  flexibility  in  swimming. 

Subsequently,  at  the  beginning  of  the  Era  of  Mammals, 
some  of  the  river-turtles  left  the  water  for  the  land  and 
developed  the  complete  rigid  shells  of  the  tortoises. 

This  view  of  the  development  of  the  turtles  has  not 
been  proved  by  geology ;  for  the  river-dwelling  mud- 
turtles  have  only  been  found  in  the  upper  part  of  the 
Chalk  Period,  whereas  the  sea-turtles  are  known  earlier 
than  the  beginning  of  that  Period;  but  probably  the 
river-turtles,  owing  to  their  primitive  characters,  had 
been  in  existence  long  before  the  date  of  their  known 
fossils,  but  they  have  not  yet  been  discovered,  as  river 
deposits  are  much  rarer  than  beds  laid  down  in  the  sea. 

The  last  of  the  Reptiles,  the  Pterosauria  (wing-lizards), 
are  the  most  remarkable  of  them  all ;  for  they  had  powers 
of  flight,  and  throughout  the  Era  of  Reptiles  they  ruled 
the  air,  while  their  allies,  the  Dinosaurs,  were  supreme  on 
land.  The  oldest  of  these  flying  reptiles,  Dimorphodon, 
was  found  in  the  Lower  Lias  of  Dorset.  It  had  a  large 
skull  and  long  tail.  The  front  limbs  acted  as  wings; 

262 


Ancient  Reptiles  and  Origin  of  Birds 

each  had  four  fingers,  of  which  the  outermost  was 
enormously  elongated  and  was  bent  back  to  support  a 
membrane  like  the  wing  of  the  bat ;  and  by  its  flapping 
the  reptile  could  glide  through  the  air. 

These  flying  reptiles  lived  in  the  Jurassic  and  Cretaceous 
Periods ;  those  of  the  earlier  Period,  including  the  Ptero- 
dactyl, were  small,  and  some  were  no  larger  than  a 
sparrow,  and  they  could  probably  fly  as  well  as  bats.  The 
later  flying  reptiles  were  much  larger,  and  the  great 
Pteranodon  (winged  and  toothless)  of  Nebraska  had  an 
expanse  of  wing  of  eighteen  feet.  These  animals  were 
lightly  built,  and  had  hollow  bones ;  but  it  is  very  doubtful 
whether  so  great  an  animal  as  Pteranodon  could  really  fly. 
Its  wings  were  less  efficient  than  those  of  bats,  in  which 
the  membrane  is  supported  by  four  ringers,  whereas  in 
Pteranodon  and  the  flying  reptiles  it  was  supported  only 
by  one  finger,  and  would  hardly  have  been  sufficiently 
rigid  for  rapid  or  effective  action  as  a  wing.  Pteranodon 
had  a  much  larger  wing  expanse  than  the  albatross,  but 
had  a  heavier  weight  to  carry ;  and  probably  all  it  could 
do  was  to  glide  through  the  air  from  cliffs  or  trees  up 
which  it  climbed  with  its  hooked  claws. 

These  flying  reptiles  were  not  the  ancestors  of  birds,  for 
they  descended  from  Dinosaurs.  Birds  differ  from  reptiles 
by  being  bot-blooded;  but  they  agree  in  the  significant 
character  that  the  skull  is  attached  to  the  backbone  by 
one  joint-surface,  and  not  two,  as  in  amphibians  and 
mammals ;  and  in  several  other  ways  the  organization  of 
birds  and  reptiles  is  closely  allied. 

The  birds  probably  developed  from  the  group  of  Dino- 
saurs in  which  the  structure  of  the  legs  and  hip-girdle  are 
strikingly  bird-like.  The  actual  origin  of  birds  is  uncertain. 
According  to  one  suggestion  a  Dinosaur  adopted  an 
aquatic  life,  and  used  its  fore-limb  as  a  flapper,  and  in- 
creased the  area  of  this  limb  by  hairy  quills,  which  were 

263 


Ancient  Reptiles  and  Origin  of  Birds 

at  first  like  the  spine  of  the  porcupine  ;  then  the  outer 
layer  of  these  spines  was  uncurled  into  a  vane,  and  the 
quill  was  thus  converted  into  the  feather.  This  sugges- 
tion of  the  aquatic  origin  of  the  birds  is  rendered  improb- 
able from  the  nature  of  the  oldest  known  bird,  the 
Archseopteryx.  This  was  a  small  bird  about  the  size  of 
a  pigeon,  of  which  two  skeletons  have  been  found  in  the 
Solenhofen  Slate  of  Bavaria.  Its  skeleton  in  several 
respects  resembles  that  of  reptiles,  and  had  not  the 
feathers  been  preserved  it  might  have  been  regarded 
as  a  kind  of  lizard.  Its  wings,  however,  were  feathered, 
and  it  had  a  long  tail,  in  which  each  joint  bore  a  pair 
of  feathers.  This  animal  differed  from  all  modern  birds 
by  the  existence  of  this  long  tail,  and  by  having  teeth 
instead  of  a  beak.  In  all  other  known  birds  the  tail  has 
been  shortened,  and  all  the  bones  in  it  have  grown 
together  to  form  the  ploughshare  bone  ;  and  from  this 
bone  the  tail  feathers  arise  in  a  tuft,  as  is  well  seen  in  the 
fantailed  pigeon.  In  the  absence  of  the  tuft  and  the 
structure  of  the  tail,  Archaeopteryx  is  therefore  primitive, 
and  resembles  the  reptiles.  It  agrees  with  them,  moreover, 
in  the  fact  that  each  wing  had  three  separate  fingers 
ending  in  a  claw.  If  Archaeopteryx  was  the  real  ancestor 
of  the  birds,  they  were  probably  descended  from  some 
reptile,  which  lived  in  trees  and  had  developed  its  hairs 
into  quills  for  defensive  purposes ;  it  may  then  have  found 
that  when  the  quills  were  held  horizontally,  they  aided 
it  in  gliding  from  tree  to  tree ;  and  the  quills  were 
developed  into  feathers  by  the  spreading  out  of  the  outer 
case  to  form  the  vane.  This  development  greatly  increased 
the  wing  area,  and  when  the  thin  barbs  interlock  they 
form  a  broad,  firm  surface ;  if  this  strikes  the  air  quickly, 
the  air  resists  like  a  solid  ;  so  by  a  quick  downstroke  and 
slow  uplift  of  the  wings,  birds  rise  through  the  air. 

The  birds  of  the  Chalk  Period,  and  even  those  of  the 

264 


Ancient  Reptiles  and  Origin  of  Birds 

beginning  of  the  Era  of  Mammals,  had  teeth ;  but  these 
in  time  were  replaced  by  a  horny  beak. 

Flying  birds  are  limited  to  a  comparatively  small  size ; 
those,  like  ostriches  and  emus,  which  lost  the  power  of 
flight,  freed  from  its  limitations,  grew  much  larger.  The 
most  remarkable  of  fossil  flightless  birds  are  the  Moas  of 
New  Zealand,  which  are  now  extinct,  but  were  abundant 
when  the  Maoris  first  reached  New  Zealand  about  six 
hundred  years  ago.  Until  then,  the  Moas  had  no  serious 
enemies ;  they  inhabited  an  island  rich  in  food,  and  as 
there  were  no  flesh-eating  animals  to  attack  them,  they 
found  flight  unnecessary,  adopted  the  habit  of  living  on 
the  ground,  and  multiplied  in  numbers  and  varieties.  They, 
however,  fell  victims  to  the  Maori,  who  killed  them  for 
food  and  feathers.  Some  of  them  may  have  been  alive 
when  Captain  Cook  visited  New  Zealand.  A  fragment 
of  a  bone  was  sent,  in  1839,  t°  Prof.  Owen,  who  recognized 
that  it  belonged  to  a  giant  bird,  and  this  conclusion 
was  confirmed  by  the  discovery  of  complete  skeletons 
with  the  feathers,  sinews,  and  even  clots  of  blood. 

The  largest  of  the  Moas  were  about  eleven  feet  high ; 
and  other  large  extinct  flightless  birds  lived  in  Australia 
and  India.  The  ^pyornis  of  Madagascar  was  a  bird 
belonging  to  the  same  Group,  and  it  has  the  largest  of 
known  eggs ;  the  egg  has  a  capacity  of  over  two  gallons 
and  about  six  times  as  large  as  that  of  the  ostrich ;  and  if 
the  bird  had  been  proportionately  larger  it  would  have 
been  the  most  titanic  of  birds.  The  legend  of  the  Roc 
which  flew  away  with  Sindbad  the  Sailor  was  probably 
based  on  the  discovery  of  these  colossal  eggs  and  the 
expectation  that  birds  of  corresponding  size  lived  in  the 
islands  of  the  Indian  Ocean. 

1  The  term  "  Dinosaur  "  is  used  in  its  older,  wider  sense  to  include 
the  carnivorous  forms  and  their  herbivorous  allies ;  the  latter  are 
now  often  separated  as  the  Orthopoda  (straight-foot),  a  name  proposed 
by  Cope. 

265 


CHAPTER  XIX 
THE  EVOLUTION  OF  MAMMALS 

THE  mammals,  as  previously  remarked  (p.  254),  were  prob- 
ably developed  from  the  reptiles  known  as  Theromorphs  ; 
and  this  momentous  event  in  the  evolution  of  life  hap- 
pened at  the  beginning  of  Mesozoic  times,  and  apparently 
on  the  great  continent  of  Gondwanaland  in  what  is  now 
South  Africa.  At  least,  fossils  found  in  South  Africa  have 
yielded  the  best  evidence  as  to  the  gradual  development 
of  reptiles  into  mammals.  The  essential  differences  be- 
tween the  two  groups  are  that  the  reptiles  are  cold-blooded 
and  the  mammals  warm-blooded ;  the  reptiles  usually 
have  scaly  coats,  whereas  the  mammals  have  a  leathery 
hide  or  skin,  which  is  covered  with  hair  ;  the  head  of  the 
reptiles  works  on  the  backbone  by  only  one  joint-surface, 
whereas  in  the  mammals  there  are  two  joint-surfaces  ;  and 
the  brain  of  the  reptile  is  much  smaller  than  that  of 
mammals  of  corresponding  size.  The  mouth  of  the 
reptile  has  a  much  wider  gape  to  enable  it  to  swallow 
its  food,  whereas  in  the  mammals  the  jaw  is  more  loosely 
hung,  so  that  it  can  grind  and  masticate  its  food,  but  the 
mouth  cannot  be  opened  as  widely. 

The  gradual  change  from  the  reptile  to  the  mammal 
has  been  shown  by  Dr.  Broom  from  the  rich  collection  of 
fossils  discovered  in  South  Africa.  In  Lower  Permian 
times  some  of  the  reptiles  that  lived  in  Gondwanaland 
acquired  teeth  like  those  of  the  mammals ;  they  required 
both  to  cut  and  to  grind  their  food,  so  the  front  teeth 

266 


The  Evolution  of  Mammals 

were  adapted  for  cutting  and  the  back  teeth  for  grinding. 
This  change  was  gradually  followed  by  others  in  the 
skull,  and  especially  the  development  of  a  mammal-like 
palate.  Some  of  these  reptiles  with  mammal-like  teeth 
fed  on  plants,  and  others  were  carnivorous  and  fed  upon 
the  plant-eating  reptiles. 

These  were  at  first  small  and  sluggish,  and  such  an 
easy  prey  that  they  were  exterminated.  The  carnivorous 
reptiles  had  then  to  give  chase  to  more  active  animals. 
One  result  was  an  important  change  in  the  support  of  the 
skull.  In  the  Triassic  Cynodonts  the  one  joint-surface, 
by  which  the  head  was  moved  on  the  backbone,  gradually 
subdivided,  and  the  two  divisions  ultimately  separated 
into  two  distinct  joint-surfaces,  as  in  the  mammals.  Such 
Cynodonts  could  not  have  spun  their  heads  round  as 
quickly  as  reptiles  or  birds,  but  for  general  purposes  they 
had  acquired  a  far  more  useful  attachment  of  the  head  to 
the  body.  These  Cynodonts  multiplied  and  flourished  by 
eating  up  the  herbivorous  reptiles  and  many  smaller  ones, 
which  resembled  small  lizards,  and  were  comparatively 
defenceless.  The  Cynodonts  in  the  lower  part  of  the 
Jurassic  were  therefore  forced  by  the  disappearance  of 
their  first  forms  of  prey  to  chase  longer-legged  and  more 
active  animals,  such  as  the  Thecodonts  (Gr.  theke>  a  case  ; 
odous,  a  tooth),  animals  with  teeth  in  distinct  sockets. 
The  necessity  for  speed,  endurance,  and  skill  in  this  chase 
led  to  their  evolution  through  a  series  of  stages  which 
converted  them  into  mammals ;  but  the  process  was  so 
gradual  that  it  is  not  yet  known  where  to  draw  the  line 
between  the  mammal-like  reptiles  and  reptile-like  mam- 
mals. They  dropped  their  scaly  armour,  which  was  re- 
placed by  a  plastic  skin ;  their  active  life  led  to  the  blood 
becoming  warmer,  and  this  change  required  the  alteration 
of  the  three-chambered  heart  of  the  reptiles  into  the  four, 
chambered  heart  of  the  mammals,  and  as  protection 

267 


The  Evolution  of  Mammals 

against  cold  was  then  necessary,  this  change  was  accom- 
panied by  the  growth  of  a  hairy  fur.  The  constant  need 
for  careful  observation  and  alertness  in  action,  by  which 
alone  the  Cynodonts  could  obtain  an  adequate  supply  of 
their  fleet  prey,  stimulated  their  intelligence,  and  led  to  a 
great  development  in  the  size  of  the  brain. 

The  most  important  of  all  the  remarkable  changes 
which  happened  during  the  passage  from  Palaeozoic  to 
Mesozoic  times  was  the  evolution  of  the  sluggish,  cold- 
blooded, scaly  reptile  to  the  alert,  warm-blooded,  fur-clad, 
intelligent  mammal. 

The  most  primitive  of  the  living  mammals  are  the 
Australian  Monotremes  (single  -  holed  :  Gr.  monos,  one; 
trema,  a  hole),  such  as  the  duck-billed  Platypus,  which, 
among  other  resemblances  to  the  reptiles,  laid  eggs.  The 
other  primitive  group  of  mammals  is  that  of  the  marsupials, 
or  pouched  mammals,  which  are  also  characteristic  of 
Australia.  The  young  marsupials  are  born  in  a  very 
immature  condition,  and  are  carried  by  the  mother  in 
a  pouch  until  they  have  grown  enough  to  run  about  inde- 
pendently. A  few  marsupials,  such  as  the  opossums,  live 
in  America ;  there  are  none  in  Europe,  Asia,  or  Africa. 
They  reach  their  highest  development  in  Australia,  where 
they  are  represented  by  the  kangaroos,  wallabies,  wom- 
bats, and  Australian  moles,  bears,  and  opossums.  In 
Australia  all  the  native  mammals  are  marsupials,  except 
some  bats,  rats,  and  the  dingo,  which  may  have  been 
introduced  on  driftwood  or  by  man.  None  of  the  higher 
orders  of  mammals,  such  as  deer,  cattle,  elephants,  cats, 
bears,  wolves,  squirrels,  horses,  monkeys,  rabbits,  or  sloths, 
have  ever  lived  in  Australia.  Yet  the  country  would 
have  been  well  suited  for  them,  so  that  Australia  must 
have  been  isolated  from  all  the  other  lands  since  the 
appearance  on  earth  of  any  mammals  higher  than 
marsupials. 

268 


The  Evolution  of  Mammals 

The  marsupials  and  egg-laying  mammals  both  lived  in 
Europe  in  the  Era  of  Reptiles,  but  they  were  always 
scarce  and  small,  as  they  were  then  too  weak  to  compete 
successfully  with  the  giant  reptiles.  The  teeth  of  these 
primitive  mammals  have  been  found  in  the  Trias  of  Eng- 
land, Germany,  and  South  Africa;  and  their  jaws  have 
been  found  in  some  numbers  in  the  Stonesfield  Slate  of 
Oxfordshire,  and  in  the  Isle  of  Purbeck.  The  beds  con- 
taining the  two  chief  deposits  of  early  fossil  mammals 
both  belong  to  the  Jurassic.  In  the  Cretaceous  Period 
we  might  have  expected  still  more  numerous  mammals  in 
the  land  and  freshwater  beds  of  the  Weald  of  Kent ;  but 
in  spite  of  prolonged  search  only  two  or  three  mammal 
teeth  have  been  found  there,  and  it  appears  as  if  the 
mammals  had  been  almost  exterminated  by  the  reptiles. 
The  marsupials  were  exterminated  in  the  Old  World  in 
the  beginning  of  the  Kainozoic  Era  by  the  rapid  evolution 
of  the  higher  mammals,  and  by  this  time  the  marsupials 
were  in  occupation  of  Australia,  where,  free  from  compe- 
tition, they  increased  in  number  and  variety.  Various 
members  of  this  one  order  adopted  different  habits,  and 
some  were  modified  into  the  tree-living  opossums,  squirrels, 
and  flying  foxes.  The  kangaroos,  wallabies,  and  Aus- 
tralian hares  developed  powers  of  swift  running,  and  lived 
on  the  open  plains :  one  burrowed  into  the  ground  and 
acquired  the  shape  and  habits  of  the  mole ;  another  took 
to  life  on  the  floor  of  dense  forests  and  became  the  heavy, 
powerful  wombat ;  others  living  in  the  trees  became  the 
Australian  bears ;  and  others  finding  such  an  abundance 
of  herbivorous  animals  took  to  preying  on  tfiem,  and 
acquired  the  forms  of  cats,  wolves,  and  dogs.  Some  of 
the  marsupials  also  grew  to  a  large  size,  such  as  the  giant 
Diprotodon  (double  front-toothed  :  Gr.  dis,  twice ;  protos, 
first  or  front),  which  was  as  big  as  a  rhinoceros,  and 
browsed  on  the  vegetation  around  the  former  Australian 

269 


The  Evolution  of  Mammals 

lakes.  The  giant  kangaroos  were  twice  as  large  as  the 
tallest  "  old  man  kangaroo  "  still  living  in  Australia,  and 
these  herbivorous  marsupials  were  preyed  upon  by  the 
marsupial  wolf.  Why  these  large  animals  became  ex- 
tinct is  unknown.  They  were  too  large  to  have  been 
exterminated  by  the  killing  of  the  adults  by  any  car- 
nivorous beast  in  the  country ;  but  they  certainly  survived 
till  the  arrival  of  the  dingo,  and  the  packs  of  these  wild 
dogs  may  have  harassed  the  adults  and  devoured  the 
young.  The  disappearance  of  the  giant  marsupials  from 
parts  of  the  interior  of  Australia  may  be  explained  by 
a  change  of  climate,  and  the  drying  up  of  former  lakes. 
But  this  explanation  will  not  apply  to  the  fertile  plains 
of  Victoria,  where  there  has  been  no  desiccation  of  the 
country,  and  the  large  marsupials  might  have  been  ex- 
pected to  survive,  as  the  smaller  ones  have  done.  Possibly 
increase  in  the  size  of  the  herds  of  kangaroos  at  a  time  of 
decreasing  food-supply  led  to  the  slow  reduction  in  size, 
and  to  the  extinction  of  those  like  the  Diprotodon,  which 
for  some  reason  did  not  decrease  in  size. 

While  the  pouched  mammals  were  developing  in 
Australia,  a  higher  type  of  mammals  was  being  evolved  in 
the  lands  of  the  Northern  Hemisphere ;  and  its  evolution 
is  the  most  important  event  in  the  Kainozoic  Era,  there- 
fore named  the  "  Era  of  Mammals." 

The  first  epoch  in  this  Era  is  known  as  the  Eocene,  or 
"  dawn  of  recent  life  " ;  and  during  it  there  appeared  the 
ancestors  of  the  different  existing  groups  of  higher 
mammals.  All  the  mammals  then  living  were  simple  in 
character,  and  they  resembled  one  another  more  than  do 
their  descendants  to-day.  Thus,  at  the  present  time,  the 
flesh-eating  animals  which  have  clawed  feet,  are  very 
unlike  the  vegetarian  hoofed  animals  on  which  they  prey ; 
yet  both  groups  can  be  traced  back  to  one  ancestor.  This 
animal  was  nearly  allied  to  a  small  primitive  beast  named 

270 


The  Evolution  of  Mammals 

Phenacodus,  which  lived  in  Wyoming  and  Switzerland.  It 
had  a  small  elongated  head  with  a  tiny  brain,  and  broad 
feet  each  with  five  toes ;  it  walked  on  the  toes  with  the 
heel  raised  off  the  ground.  In  a  later  part  of  the  Eocene 
lived  an  allied  animal,  the  Hyracotherium  (mouse-like 
beast :  Gr.  humx,  a  mouse,  and  therion,  diminutive  of  them, 
beast),  in  which  the  legs  were  longer  and  more  adapted 
for  running,  as  it  walked  on  the  tips  of  its  toes.  This 
change  in  the  use  of  toes  was  accompanied  by  a  reduction 
in  their  number ;  there  were  four  toes  on  each  front-leg 
and  three  on  each  hind-leg.  The  descendants  of  Hyraco- 
therium carried  this  reduction  of  toes  still  further  until 
the  Miocene  Mesohippus  had  only  three  well-developed 
toes  on  each  foot.  The  next  stage  was  the  decrease  of 
two  outer  toes,  until  in  the  Pliocene  Hipparion  they  only 
touched  the  ground  when  the  animal  was  walking  through 
swamps  or  soft  sand  or  snow ;  on  hard  ground  the  animal 
stood  only  on  the  middle  toe  of  each  foot.  Hipparion  was 
the  immediate  ancestor  of  the  horse,  in  which  the  foot  has 
been  modified  until  only  one  toe  is  left.  The  foot  consists 
of  a  single  bone,  which  corresponds  to  the  one  joint  of  the 
middle  finger  or  toe  in  our  hands  or  feet.  Traces  of  the 
smaller  toes  of  Hipparion  may  be  recognized  in  the  thin 
splint  bones  which  lie  along  the  one  remaining  bone. 

The  leg  of  the  horse  corresponds  to  the  greatly 
elongated  middle  finger  or  middle  toe  of  man  and  other 
mammals.  The  joint  which  in  the  horse  is  called  the 
knee  corresponds  to  our  ankle  and  wrist.  This  arrange- 
ment is  the  result  of  the  horse  having  been  adapted  to 
fast  running  over  open  ground,  and  to  defending  itself 
from  carnivorous  animals  by  the  strength  of  its  kick ; 
and  the  hind-leg  of  the  horse,  judged  simply  as  kicking 
machinery,  is  an  excellent  piece  of  apparatus.  This 
arrangement  has  now  been  traced  backward  through  a 
long  succession  of  ancestors,  into  animals  which  had  the 

271 


The  Evolution  of  Mammals 

normal  five  toes  and  walked  on  a  flat  foot.  The  first  true 
horse  appeared  in  the  Lower  Pliocene  rocks  of  India,  and 
horses  first  entered  Europe  in  the  upper  part  of  that 
Epoch;  they  entered  America  early  in  the  succeeding 
Epoch,  and  spread  rapidly  through  both  North  and  South 
America;  then,  for  some  still  unexplained  reason,  they 
suddenly  became  extinct  in  America. 

While  the  horse  was  developing  its  one- toed  limbs 
from  a  five-toed  ancestor,  another  group  of  herbivorous 
mammals  acquired  long  legs  and  two-toed  feet,  as  in 
cattle,  antelopes,  and  deer.  And  many  of  these,  in 
addition  to  the  speed  given  them  by  their  long-running 
limbs,  acquired  an  additional  defence  against  their  carni- 
vorous foes,  from  various  forms  of  horns  and  antlers. 

Horns,  using  that  term  in  a  wide  sense,  are  of  three 
main  kinds :  there  are  bony  outgrowths  like  the  antlers  of 
the  deer,  which  are  dropped  and  grow  afresh  every  year  ; 
there  are  the  horns  like  those  of  cattle  and  antelopes, 
which  consist  of  a  layer  of  horn  growing  around  or  rising 
from  a  core  of  bone ;  and  there  are  horns  composed  of 
compact  tufts  of  hair,  or  growths  of  material  like  the 
finger-nail,  such  as  the  horn  of  the  rhinoceros. 

The  horned  animals  have  been  traced  back  to  un- 
horned  Oligocene  ancestors.  The  first  horns  were  small 
spikes,  and  they  only  began  to  branch  into  antlers  in  the 
Pliocene.  Horns  were  modified  from  mere  sharp  bayonets, 
which  were  used  in  stabbing  or  ripping  up  flesh-eating 
quadrupeds,  into  structures  useful  as  adornments,  or  for 
.fights  with  their  fellows.  These  family  fights  are  mainly 
between  the  males  for  the  possession  of  the  females,  and 
hence,  instead  of  both  males  and  females  having  horns, 
as  with  the  Oryx  that  uses  horns  for  defence  against  lions 
and  leopards,  they  are  restricted  in  the  deer  to  the  male. 
They  reached  the  greatest  development  in  the  Great  Irish 
Deer  (Cervus  giganteus),  in  which  the  antlers  were  as  much 

272 


The  Evolution  of  Mammals 

as  twelve  feet  wide  ;  they  were  probably  so  heavy  that  they 
handicapped  the  animals  when  pursued.  When  man 
reached  Ireland,  these  heavy  deer  were  probably  too 
sluggish  to  escape,  and  fell  an  easy  prey  to  the  hunters. 
It  lived  in  many  parts  of  Europe,  but  probably  survived 
last  in  Ireland,  where  its  remains  are  found  abundantly  in 
the  peat  bogs. 

Another  group  of  horned  animals  depended  for  defence 
on  their  great  bulk,  their  simple  conical  horns,  and  the 
thickness  of  their  hides.  While  the  horned  deer  and 
cattle  were  developing  long  legs  adapted  for  running, 
another  group  of  hoofed  or  ungulate  animals  was  growing 
a  massive  body,  with  skin  so  thick  that  it  would  resist 
injury  by  ordinary  teeth  and  claws,  and  a  body  so  heavy 
that  its  charge  would  knock  down  any  ordinary  carnivore, 
which  it  could  then  trample  on  and  crush  by  rolling  over 
it.  Of  these  animals  the  living  representative  is  the  rhino- 
ceros. It  is  the  result  of  a  long  line  of  evolution;  its 
Eocene  ancestor  was  an  animal  allied  to  Palseotherium, 
which  was  a  primitive  ancestor  of  the  horse.  Palaeo- 
therium  was  succeeded  by  a  hornless  animal,  the  Acera- 
therium,  which  had  a  tiny  and  useless  fourth  toe  on  the 
foot ;  and  from  this  animal  gradually  came  the  horned 
three-toed  rhinoceros  of  the  Old  World.  Meanwhile, 
another  horned  ally  of  the  rhinoceros  had  developed  in 
Egypt,  where  the  bones  have  been  found  in  the  same 
bed  as  an  ancestor  of  the  elephant  known  as  Palaeo- 
mastodon  (p.  275) ;  it  was  a  remarkable  beast,  named  by 
its  discoverer,  Mr.  Beadnell,  Arsinotherium  (Arsinoe's 
little  beast,  so  named  because  it  was  found  at  the  tomb 
of  the  Egyptian  princess,  Arsinoe).  Its  skull  was  three  feet 
long,  and  from  it  rose  two  short,  thick  pointed  horns, 
placed  side  by  side,  instead  of  one  in  front  of  the  other, 
as  in  the  rhinoceros.  Arsinotherium  had  thick  legs  and 
five  toes,  and  many  teeth  ;  it  had  no  direct  descendants, 

273  s 


The  Evolution  of  Mammals 

but  a  somewhat  similar  animal  appeared  about  the  same 
time  in  North  America.  It  was  the  Dinoceras  (terribly 
horned),  which  had  six  blunt  horns  on  the  top  of  the 
skull,  with  which,  by  a  sudden  upward  thrust,  it  could 
no  doubt  have  killed  or  seriously  injured  any  animal  that 
attacked  it.  Dinoceras  resembled  a  rhinoceros  in  size 
and  in  other  respects  more  than  did  Arsinotherium.  Still 
more  allied  to  the  rhinoceros  was  the  huge  Titanotherium, 
from  the  Lower  Miocene  beds  of  Dakota ;  it  was  larger 
than  the  rhinoceros,  but  its  horns  were  probably  similar 
in  structure,  for  they  were  probably  compacted  tufts  of 
hair  or  a  nail-like  structure  supported  on  a  thickened 
knob  of  bone.  The  Titanotherium  (gigantic  beast ;  Titan, 
one  of  the  giants  who  fought  against  the  new  gods  under 
Jupiter)  had  a  pair  of  these  horns  side  by  side  on  the 
front  of  the  head,  and  its  feet  were  more  primitive  than 
those  of  the  rhinoceros,  for  it  had  four  toes  on  the  front 
feet,  and  three  toes  on  the  hind  feet. 

In  still  later  times  the  true  rhinoceros  was  developed. 
One  species,  the  Woolly  Rhinoceros  (R.  antiquitatis),  lived 
in  northern  Europe  and  Asia  during  the  glacial  time,  and 
has  been  preserved  like  the  Mammoth  by  frost  in  Siberian 
swamps.  Its  bones  and  teeth  are  often  found  in  the 
gravels  of  the  Thames  Valley. 

An  allied  animal,  the  Elasmotherium  (elasma,  a  metal 
plate),  which  also  lived  in  Siberia,  was  a  rhinoceros  with  a 
skull  a  yard  long.  It  carried  one  large  horn,  which  was 
situated  on  the  forehead,  and  not  on  the  nose  as  in  the 
rhinoceros. 

1 1  The  elephant,  with  its  long  flexible  trunk,  its  huge  ivory 
\  tusks— one  of  which  in  the  Natural  History  Department 
of  the  British  Museum  is  10  feet  2  inches  long  and 
weighs  228  pounds— and  the  fewness  of  its  teeth,  is  among 
the  most  remarkable  of  all  mammals.  There  are  two 
living  kinds— the  African  and  the  Indian.  The  African 

274 


The  Evolution  of  Mammals 

is  the  larger  of  the  two,  and  is  probably  as  massive  and  as 
heavy  as  any  animal  that  has  ever  lived  on  dry  land.  The 
ancestry  of  the  elephant  was  for  long  unknown,  but  has 
been  revealed  by  the  discoveries  of  Dr.  C.  W.  Andrews 
in  the  Fayum,  a  basin  to  the  south-west  of  Cairo  in 
Egypt.1 

He  there  found  the  remains  of  a  big  pig-like  animal 
vhich  he  called  the  Mceritherium.  It  had  a  long  skull, 
nd  its  teeth  were  numerous  and  varied  in  form,  as  in 
nost  mammals.  It  had  on  each  side  of  each  jaw  six 
molar  or  grinding  teeth,  and  in  front  six  cutting  or  incisor 
eeth  in  the  upper  and  four  in  the  lower  jaw,  and  a  pair  of 
arge  tusk-like  or  canine  teeth  in  the  upper  jaw.  The 
kull  had  a  fleshy  snout  in  front,  and  this  may  have  been 
ufficiently  long  and  flexible  to  form  a  short  primitive 
runk.  The  feet  were  large,  with  five  blunt  toes.  The 
most  remarkable  point  about  the  Moeritherium  was  the 
ature  of  its  molar  teeth.  The  surface  of  each  of  them  is 
aised  into  a  series  of  two  or  three  blunt  points,  which  are 
attened  so  that  they  form  transverse  ridges.  These 
,dges  rendered  the  teeth  very  effective  in  crushing  and 
rinding  the  vegetation  on  which  it  fed. 
The  Moeritherium  lived  at  the  end  of  the  Eocene  times; 
ut  the  surface  ridging  of  the  teeth,  first  found  in  this 
reature,  survived  in  later  animals,  such  as  the  Mastodon, 
hich  is  a  well-developed  elephant. 

The  Moeritherium,  or  some  cousin  of  it,  developed  into 

le  oldest  of  the  Mastodons,  which  is  therefore  known 

s  Palseomastodon  (a  primeval,  round-toothed  creature: 

r.  palaios,  ancient ;  mastos,  a  woman's  breast).     It  was 

robably  the  size  of  a  horse,  and  differed  from  Mceri- 

herium  by  the  front  part  of  the  jaws  having  grown  longer 

nd  narrower.     The  number  of  crushing  teeth  had  been 

:duced  from  six  to  five  on  each  side  of  the  lower  jaw ; 

le  canine  teeth  had  been  lost,  and  the  incisors  reduced 

275 


The  Evolution  of  Mammals 

to  two  in  each  jaw,  and  the  pair  in  the  upper  jaw  had 
been  prolonged  into  tusks.  The  pair  of  incisors  in  the 
lower  jaw  were  smaller,  and  formed  a  flat  projection  that 
probably  supported  the  long  flexible  upper  lip. 

After  this  period,  though  the  Mediterranean  Sea  was 
already  in  existence,  there  were  land  connections  across  it 
to  southern  Europe.  The  descendants  of  Palseomastodon 
crossed  from  northern  Africa  to  southern  Europe,  and 
lived  in  Miocene  times  in  the  south  of  France.  The 
changes  between  Mceritherium  and  Palseomastodon  made 
further  progress.  The  number  of  crushing  teeth  was 
reduced  in  the  adult  to  two  pairs  in  each  jaw,  and  their 
reduction  in  number  was  compensated  by  their  increase 
in  size,  and  in  the  strength  of  the  ridges  on  their  surfaces. 
The  lower  jaw  became  still  longer  and  thinner;  its  pair  of 
tusks  still  projected  forward ;  the  upper  jaw  became  much 
shorter,  while  its  tusks  increased  in  size  and  were  sepa- 
rated ;  the  space  between  them  was  occupied  by  a  long, 
flexible  upper  lip,  which  had  fused  with  the  soft  parts  of 
the  nose,  and  the  two  formed  a  well-developed  trunk, 
which  was  about  as  long  as  the  rest  of  the  head,  but 
ended  in  front  level  with  the  lower  jaw. 

This  animal,  Tetrabelodon  (four-tusked),  was  an  ele- 
phant with  a  long  lower  jaw.  Its  evolution  into  the 
modern  elephant  was  achieved  by  three  main  changes: 
first,  the  lower  jaw  was  shortened,  the  lower  front  teeth 
were  lost,  and,  with  their  disappearance,  the  projection 
of  the  lower  jaw  disappeared,  and  left  it  as  the  short  jaw 
of  the  elephant ;  second,  the  tusks  on  the  upper  jaw  were 
greatly  increased  in  size  and  strength,  and  pointed  out- 
ward, and  thus  became  of  increased  use  to  the  animal  b) 
enabling  it,  like  the  modern  elephant,  to  dig  up  the  dr} 
river-beds  during  times  of  drought  and  thus  obtain  water 
and  at  the  same  time  as  the  tusks  grew  longer  the  bone 
of  the  face  were  shortened,  giving  the  skull  the  flat-face< 

276 


The  Evolution  of  Mammals 

form  of  the  elephant.  Sir  Ray  Lankester  has  aptly  com- 
pared the  process  by  which  the  long-faced  skull  of  Mceri- 
therium  and  Palaeomastodon  was  shortened  into  that 
of  the  elephant  to  the  change  from  the  long  skull  of  the 
greyhound  to  that  of  the  bulldog,  and  has  called  this 
shortening  of  the  face  the  "  bulldogging  "  of  the  skull. 
In  consequence  of  the  shortening  of  the  lower  jaw,  the 
long  upper  lip  was  left  unsupported ;  it  therefore  drooped 
between  the  two  tusks,  and,  increasing  in  length  and 
flexibility,  became  the  trunk. 

The  last  changes  which  completed  the  evolution  of  the 
elephant  was  in  the  molars.  The  Mastodon  had  as  few  as 
two  or  three  molars  on  each  side  of  both  jaws,  and  every 
tooth  had  three  to  six  transverse  ridges.  The  teeth  are  not 
all  present  together ;  they  grow  in  succession,  and  as  the 
younger  tooth  increases  in  size,  its  predecessor  is  pushed 
forward  and  lost.  But  in  the  fully  adult  elephant  of  our 
day  there  is  only  one  molar  tooth  on  each  side  of  each  jaw. 
The  single  tooth  of  the  elephant,  however,  does  even  more 
efficiently  than  the  pair  of  teeth  in  the  Mastodon,  for  the 
surface  is  crossed  by  a  series  of  transverse  ridges ;  of  these 
there  are  as  many  as  eleven  in  the  African  and  twenty- 
seven  in  the  Indian  species,  and  these  ridges  enable  the 
elephant  to  crush  the  coarse  grasses  and  reeds,  which  are 
its  main  food. 

The  Indian  elephant  is  doubtless  descended  from  the 
Mastodons,  and  is  closely  related  to  the  extinct  hairy 
elephant,  the  Mammoth  (Elephas  primigenius),  which  was 
larger  than  the  Indian,  but  not  larger  than  the  African 
elephant.  It  was,  however,  more  closely  related  to  the 
Indian  elephant  through  the  structure  of  the  teeth.  In 
the  African  elephant  the  ridges  on  the  teeth  are  lozenge- 
shaped,  and  up  to  eleven  in  number ;  the  Mammoth  and 
Indian  elephant  agree  in  having  a  larger  number  of  ridges, 
which  are  long,  narrow,  and  parallel-sided. 

277 


The  Evolution  of  Mammals 

The  mammoth  is  now  extinct,  but  it  lived  in  Europe, 
northern  Asia,  and  North  America,  and  was  quite  common 
in  the  Thames  Valley,  while  large  herds  of  them  roamed 
over  the  plains  that  are  novV  the  floor  of  the  North  Sea,  as 
their  teeth  are  frequently  dredged  up  by  fishermen,  often 
to  the  detriment  of  their  trawl-nets.  It  lived  in  this 
country  after  the  Glacial  Period,  when  the  climate  was 
much  colder  than  it  is  to-day,  and  it  had  a  hairy  coat. 
The  mammoth  was  certainly  contemporary  with  man  in 
southern  France  and  Switzerland,  for  pieces  of  mammoth 
tusk  have  been  found  on  which  some  early  artist  has 
engraved  a  picture  of  the  mammoth.  The  most  famous 
of  these  drawings  was  found  in  the  cave  of  the  Dordogne 
in  southern  France,  and  is  a  very  skilful  engraving.  It 
showed  the  particular  curve  of  the  tusks  found  in  the 
mammoth,  its  blunt,  massive  head,  and  many  fine 
vertical  lines  cut  to  represent  the  hair.  That  the  mam- 
moth was  hairy  has  been  proved  by  discoveries  in  Siberia, 
where  many  mammoths  have  been  found  buried  in  swamps, 
and  been  so  frozen  that  the  skin  and  hair,  and  even 
the  flesh,  have  been  preserved.  From  this  circumstance 
rumours  have  spread,  sometimes  due  to  an  attempted 
hoax,  sometimes  to  natives  describing  to  one  party  of 
explorers  pictures  shown  them  by  another,  that  living 
mammoths  had  been  seen  in  Siberia. 

The  ancestry  of  the  elephant  has  therefore  been  dis- 
covered, and  we  know  there  were  many  off-branches  from 
the  main  line  which  failed  in  the  struggle  for  existence, 
and  therefore  left  no  descendants.  Thus  the  Miocene 
Dinotherium  (terrible  beast :  Gr.  deinos,  strange ;  therion,  a 
beast)  had  numerous  teeth  like  those  of  a  Mastodon ;  and 
it  had,  like  the  elephant,  a  pair  of  tusks  pointing  down- 
ward ;  but  these  tusks  were  in  the  lower  jaw  instead  of 
in  the  upper.  Dinotherium  was  probably  a  descendant 
of  Palaeomastodon,  and  was  formed  from  it  by  the  two 


The  Evolution  of  Mammals 

canines  of  the  lower  jaw  being  bent  downward  so  that  the 
face  was  shortened,  and  the  upper  lip,  being  left  unsup- 
ported, sagged  down  into  a  trunk. 

During  the  evolution  of  the  main  line  of  the  elephants 
there  was  a  steady  increase  in  size ;  but  the  members  of 
one  interesting  offshoot  gradually  became  smaller  until 
they  were  reduced  to  pigmy  elephants  no  bigger  than 
dogs.  Their  remains  have  been  found  on  the  island  of 
Malta.  At  the  same  time  this  process  was  affecting  other 
African  animals  which  had  also  become  imprisoned  on 
the  lessening  islands  of  the  Mediterranean ;  for  bones  of 
pigmy  hippopotami  are  found  in  Crete  and  Cyprus. 

The  reduction  in  size  of  these  animals  was  due  to  de- 
crease in  the  food-supply  as  the  lands  became  smaller  or  to 
inbreeding.  The  herds  were  first  free  to  roam  over  land 
that  once  extended  from  the  mainland  of  Africa  to  Italy 
and  Greece ;  land  subsidence  then  separated  large  tracts  as 
islands,  in  which  herds  of  animals  were  imprisoned.  These 
islands  were  reduced  in  size  by  further  subsidences.  By 
the  crowding  of  the  animals  into  so  diminished  a  space 
the  larger  ones  were  handicapped  in  obtaining  food,  while 
the  smaller  individuals  more  easily  survived ;  and  conse- 
quently the  race  dwindled  in  size. 

From  the  history  of  the  elephants  it  can  be  understood 
how  the  development  of  some  mammals  has  been  greatly 
influenced  by  changes  in  the  distribution  of  land  and  water. 
Thus  a  successful  race  of  mammals  spreads  through  all 
the  lands  to  which  it  has  direct  access,  and  which  suit 
it  climatically.  Land  animals  therefore  spread  rapidly 
eastward  or  westward  until  their  progress  was  stopped  by 
the  sea,  and  it  is  clear  from  the  migration  of  land  animals 
that  North  America  must  have  been  repeatedly  joined  to 
Europe.  In  the  lower  part  of  the  Eocene  the  North 
American  mammals  were  so  similar  to  those  of  Europe 
and  Asia  that  the  continents  must  have  been  then  part  of 

279 


The  Evolution  of  Mammals 

one  continuous  land,  and  formed  one  zoological  province — 
Holarctica.  But  in  the  Middle  and  Upper  Eocene  the 
formation  of  the  North  Atlantic  Ocean  began  by  great 
subsidences  between  Europe  and  America.  Any  land 
connection  between  the  Old  and  New  Worlds  then  lay 
too  far  to  the  north  for  the  passage  of  ordinary  mammals. 
Hence  the  two  areas  developed  into  two  distinct  zoological 
provinces,  the  Neoarctic,  North  America,  and  the  Palseo- 
arctic,  which  included  Europe  and  Asia.  In  the  next 
period,  the  Oligocene,  began  the  great  period  of  mountain 
formation,  when  the  Alpine- Himalayan  Mountain  System 
was  formed.  The  chief  mountain-chains  of  this  system 
extend  east  and  west,  and  among  the  earliest  to  be  formed 
was  the  chain  of  the  Pyrenees ;  and  probably  a  great  uplift 
occurred  west  of  the  Pyrenees,  and  the  land  then  raised 
connected  southern  Europe  to  some  opposite  part  of  North 
America.  The  result  was  that  Europe  and  North  America 
again  formed  one  zoological  province.  The  Neoarctic  and 
Palaeoarctic  provinces  were  again  merged  in  the  Holarctic. 
In  the  Miocene  the  land  connection  across  the  Atlantic 
was  still  in  existence,  and  it  was  probably  enlarged  by 
further  movements  connected  with  the  elevation  of  the 
Alpine  System.  This  mountain-building  broke  up  the 
Mediterranean  Sea  into  a  series  of  separate  seas,  and 
the  mammals  which  had  developed  in  Africa,  such  as  the 
ancestors  of  the  Elephants,  Rhinoceros,  and  Antelopes, 
invaded  southern  Europe,  and  thence  reached  North 
America.  At  the  end  of  the  Miocene,  southern  Europe 
and  the  United  States  were  separated  by  the  formation 
of  the  North  Atlantic,  which  was,  however,  still  bounded 
to  the  north  by  land  from  Scandinavia  to  Greenland,  as 
is  shown  by  the  fact  that  the  flowering  plants  of  Greenland 
are  European  and  not  American.  This  northern  route 
was  too  cold  and  perhaps  too  interrupted  by  vast  swamps 
to  be  a  suitable  migration  route  for  the  higher  mammals, 

280 


THE  MEGATHERIUM,  A  GIANT  SLOTH 

This  animal  is  only  recently  extinct.  It  lived  in  South  America,  and  was  about  20  feet 
long  It  is  seen  using  its  long  tongue  to  tear  the  leaves  from  a  branch  and  when  necessary 
could  break  off  the  branches  or  uproot  young  trees  and  then  feed  on  the  foliage. 


The  Evolution  of  Mammals 

and  thus  those  of  North  America  and  Europe  developed 
on  different  lines.  Finally,  in  the  Pleistocene,  many  of  the 
old  mammals  in  North  America,  including  the  Mastodon, 
became  extinct,  and  by  some  route,  at  present  uncertain, 
North  America  was  again  invaded  by  the  mammals  of  the 
Old  World. 

Europe,  Asia,  Africa,  and  North  America  have  there- 
fore had  constant  or  repeated  opportunities  for  the  com- 
mingling of  their  faunas ;  but  the  continents  of  South 
America  and  Australia  have  been  more  isolated. 

There  can  be  little  doubt  that  in  various  times  Africa 
has  been  united  by  land  to  eastern  South  America  ;  since 
South  Africa  has  some  primitive  mammals  allied  to  those 
of  South  America,  there  must  have  been  a  land  connec- 
tion across  the  South  Atlantic  in  early  Kainozoic  times. 
That  land  had  been  broken  up  or  reduced  to  chains  of 
islands,  or  to  impassable  desert,  before  the  middle  part  of 
the  Kainozoic  Era,  as  otherwise  the  elephants  with  their 
great  powers  of  wandering  would  have  crossed  directly 
from  Africa  to  America,  instead  of  reaching  North  America 
through  Europe.  South  America  was  not  only  cut  off 
from  South  Africa,  but  was  isolated  from  North  America. 
Its  animals,  therefore,  developed  on  independent  lines. 
It  is  characterized  by  the  predominance  of  Edentates 
(toothless:  Lat.,  edentatus — ex,  out;  dens,  a  tooth),  so  /j\ 
named  because  of  the  absence  of  front  teeth.  _  J 

The  Edentates  include  the  sloths,  armadilloes,  and 
ant-eater.  South  America  was  also  once  inhabited  by 
many  marsupials,  or  pouched  mammals,  belonging  to  a 
group  of  marsupials  which  is  now  restricted  to  Australia 
with  the  exception  of  Ccenolestes,  which  lives  in  the 
forests  of  Ecuador.  The  marsupials,  however,  were  sur- 
passed in  South  America  by  the  Edentates,  which  grew 
to  sizes  that  are  gigantic  in  proportion  to  their  living  re- 
presentatives. The  modern  sloths  are  comparatively  small 

281 


The  Evolution  of  Mammals 

animals  which  live  in  trees ;  but  some  of  the  extinct  sloths 
were  so  large  that  no  tree  would  have  supported  them. 
They  therefore  lived  on  the  ground.  The  Giant  Ground 
Sloth,  Megatherium  (big  beast:  Gr.  megas,  big),  was  nearly 
as  large  as  an  elephant.  It  roamed  over  the  plains  of  the 
Argentine:  its  hind-legs  and  tail  were  very  massive,  so 
that  it  could  stand  erect,  using  its  front  legs  as  arms,  with 
which  it  tore  off  the  branches  of  trees  ;  it  could  probably 
also  catch  hold  of  young  trees  and  tear  them  up  by  the 
roots,  and  then  browse  on  their  foliage  (PI.  XVII.). 

Such  extravagant  habits  at  some  time  when  there  was 
a  decrease  in  the  vegetation,  may  have  led  to  the  animal 
becoming  extinct ;  but  that  explanation  is  inadequate, 
since  the  less  destructive  gigantic  sloths  and  armadilloes 
became  extinct  at  about  the  same  time,  Some  of  these 
South  American  giants  lived  till  quite  recent  times,  and 
were  contemporary  with  man.  The  Giant  Armadillo, 
Glyptodon  (Gr.  glyptos,  carved  from  the  sculptured  aspect 
of  the  teeth),  had  an  armoured  shell  like  a  turtle;  but  the 
shell  was  composed  of  a  mosaic  of  bony  plates  set  in  the 
skin.  The  largest  of  these  Glyptodon  were  fifteen  feet 
long,  and  were  several  times  as  heavy  as  the  largest  of 
modern  tortoises.  Some  of  the  smaller  ground  sloths  also 
had  a  skin  protected  by  bony  plates.  The  existence  of 
such  armour  had  been  suspected  from  the  flattening  of 
the  upper  ends  of  the  bones  which  supported  the  skin; 
but  the  fact  was  finally  proved  by  the  discovery  of  pieces 
of  skin  of  this  animal,  studded  with  the  small  round  bones. 
The  animal  was  named  Neomylodon  (new  mill-toothed), 
under  the  impression  that  it  came  from  a  ground  sloth 
that  had  survived  till  recently,  and  might  be  still  living 
in  the  forests  of  the  Andes.  More  of  the  skin  with  the 
skeletons,  and  dung,  and  even  some  of  the  soft  tissues, 
were  found  in  a  cave  beside  Last  Hope  Inlet  in  southern 
Patagonia ;  and  as  these  bones  had  been  broken  by  man, 

282 


The  Evolution  of  Mammals 

it  was  clear  that  Patagonian  Indians  lived  at  the  same 
time  as  the  Neomylodon,  and  probably  killed  the  last  of 
them  for  food. 

The  extermination  of  the  giant  Edentates  of  South 
America  has  received  no  satisfactory  explanation.  It  may 
have  been  aided  by  the  establishment  in  Middle  Pliocene 
times  of  a  land  connection  with  North  America,  for  then 
South  American  animals  entered  North  America ;  while 
South  America  was  invaded  by  many  North  American 
types,  including  the  jaguar,  the  South  American  tiger. 
The  slow  ground  sloths  were  therefore  harassed  by  the 
competition  of  herds  of  horses,  which  developed  in  South 
America  into  a  distinct  genus,  Onohippidium  (Gr.  onos, 
ass) ;  and  it  may  have  been  the  invasion  of  these  animals, 
the  killing  of  the  young  by  the  jaguars,  and  the  reckless 
slaughter  by  Indian  hunters,  which  together  led  to  their 
extermination.  The  horses,  however,  though  for  a  time 
they  had  thriven  and  multiplied,  also  became  extinct, 
though  the  country  is  climatically  well  suited  for  them, 
and  those  reintroduced  by  the  Spaniards  have  thriven 
well,  and  have  given  rise  to  herds  of  wild  horses. 

1  The  evolution  of  the  elephants  is  told  by  Dr.  Andrews  in  a 
"  Guide  to  the  Elephants,"  Natural  History  Museum,  London  (1908). 


283 


CHAPTER  XX 

THE   DRIVING   POWER  OF   EVOLUTION 

WE  have  seen  in  the  previous  chapters  that  throughout 
the  ages  there  has  been  a  gradual  evolution  of  primitive 
animals  into  the  specialized  and  more  efficient  animals  of 
to-day.  The  cause  of  this  gradual  progress  from  lower  to 
higher  types  is  a  biological  problem;  but  the  geologist 
who  recognizes  the  facts  as  to  the  actual  course  of 
evolution  is  naturally  interested  in  its  cause.  The  geo- 
logical evidence  for  a  continual  progress  in  the  develop- 
ment of  animals  and  plants  is  so  clear  that  geologists 
readily  accepted  Darwin's  explanation  of  its  cause. 
Darwinism  attributes  evolution  to  the  combined  influence 
of  a  constant  struggle  for  existence,  the  changing  of 
animals  to  adapt  themselves  to  alterations  in  the  con- 
ditions under  which  they  live,  the  extinction  of  the  in- 
adaptable  and  ill-fit,  and  the  survival  of  the  fittest. 
According  to  this  theory  evolution  is  due  to  the  selection 
of  the  most  useful  of  the  numerous  small  differences 
which  occur  between  even  closely  related  individuals. 
Animals  of  the  same  species,  then,  even  living  under 
the  same  conditions,  are  all  slightly  different  from  one 
another ;  and  those  variations  which  help  animals  in  the 
struggle  for  existence  are  preserved  and  strengthened.  In 
recent  years,  however,  many  biologists  have  rejected 
Darwinism  as  inadequate  to  explain  the  facts.  One  school 
of  naturalists  insists  that  Nature  does  not  progress  by  the 
slow  increase  of  small  changes,  but  by  sudden  jumps ; 

284 


The  Driving  Power  of  Evolution 

and  these  authorities  hold  that  animals  vary  mainly  in 
obedience  to  an  internal  inherent  impulse,  and  not  in 
answer  to  external  changes  in  their  conditions  of  life. 
This  tendency  to  vary  is  said  to  cause  the  development 
of  structures  which,  instead  of  being  useful,  are  so  exagger- 
ated as  to  be  useless,  and  perhaps  even  to  cause  the 
extinction  of  the  race.  Thus  the  colossal  antlers  of  the 
extinct  Irish  deer  may  have  rendered  it  so  sluggish  that 
it  was  an  easy  prey  to  primitive  men.  The  animal  fell  a 
victim  to  the  weight  of  its  own  armaments. 

It  is,  however,  possible  even  in  this  case  that  the  heavy 
horns  may  have  been  of  use  to  their  possessors  in  fighting. 
It  may  have  been  only  against  such  a  nimble- witted  foe 
as  man  that  the  heavy  weapons  of  the  deer  became  in- 
effective, just  as  heavily  armoured  battleships  might 
become  extinct  unless  they  could  be  defended  from  sub- 
marines and  torpedo  boats  by  a  swarm  of  alert  attendants. 

It  is  quite  true  that  no  useful  purpose  can  be  assigned 
to  many  of  the  smaller  characteristics  which  distinguish 
the  different  kinds  of  animals.  But  the  nature  of  the 
supposed  internal  impulse  towards  change  appears  not 
only  inexplicable,  but  incomprehensible. 

Dr.  Smith  Woodward  has  drawn  an  interesting  com- 
parison between  this  tendency  in  animals  and  that  of 
mineral  matter  to  grow  into  crystals.  The  crystallization 
of  a  material  may  be  hampered  by  impurities  in  it,  and 
the  attainment  of  the  regular  external  form  of  a  perfect 
crystal  may  be  impossible  owing  to  lack  of  space.  Never- 
theless the  immature  crystal  constantly  struggles  toward 
its  perfect  regular  external  shape.  It  may  remain  as  an 
irregular  grain  for  millions  of  years,  but  it  will  at  any  time 
seize  the  first  opportunity  offered  by  any  change  in  its 
conditions  to  absorb  the  surrounding  material  and  develop 
its  ideal  figure.  When  once  that  shape  is  reached,  the 
vitalism  of  the  crystal  has  achieved  its  end.  The  crystal 

285 


The  Driving  Power  of  Evolution 

henceforth  enters  a  state  of  eternal  rest.  Similarly, 
according  to  Dr.  Smith  Woodward's  analogy,  animals  are 
always  striving,  perhaps  as  unconsciously  as  the  crystal, 
to  acquire  some  particular  form ;  and  this  impulse  pro- 
duces changes  from  generation  to  generation  until  the 
ideal  is  attained.  The  impulse  then  ceases,  the  species 
changes  no  further,  and  may  become  extinct. 

The  progress  towards  special  forms  and  towards  ex- 
tinction, says  Dr.  Smith  Woodward,  "  seem  to  denote 
some  inherent  property  in  living  things  which  is  as  definite 
as  that  of  crystallization  in  inorganic  substances  "  (Rep. 
Brit.  Ass.,  1909,  p.  468). 

The  theory  that  animals  change  in  obedience  to  an 
innate  impulse  is,  however,  no  explanation  while  the 
impulse  is  unexplained.  "  We  do  not  understand  the 
phenomenon,"  remarks  Dr.  Smith  Woodward,  "  we 
cannot  explain  it "  (Rep.  Brit.  Ass.,  1909,  p.  465). 
Darwin,  on  the  other  hand,  did  offer  an  explanation  of 
the  process  by  which  evolution  is  wrought.  The  geologist 
is  therefore  interested  in  observing  whether  Darwinism 
or  the  theory  that  animals  have  developed  by  sudden 
jumps  in  obedience  to  an  inner  impulse  harmonizes  best 
with  geological  evidence  as  to  the  succession  of  life  on 
the  globe. 

In  many  cases  a  higher  fauna  is  separated  from  a 
preceding  lower  fauna  by  an  apparent  jump  ;  but  this 
break  in  the  chain  of  progress  may  be  only  due  to  some 
gap  in  the  geological  record,  and  to  the  intermediate 
forms  not  having  been  preserved. 

The  Chalk  was  laid  down  slowly  and  continuously  until 
it  reached  a  thickness  which,  even  in  its  present  com- 
pressed condition,  is  nearly  a  thousand  feet.  When  its 
fossils,  from  layer  after  layer,  have  been  carefully  com- 
pared, as  Dr.  Rowe  has  compared  its  sea-urchins,  they 
show  a  continuous  gradual  change.  This  change  may  be 

286 


The  Driving  Power  of  Evolution 

readily  explained  as  due  to  slight  differences  in  the  con- 
ditions under  which  the  animals  lived.  There  were 
numerous  changes  in  the  depth  of  the  Chalk  sea.  Some 
layers  of  chalk  cannot  have  been  formed  at  the  depth  of 
more  than  a  thousand  feet,  while  other  bands  were  laid 
down  at  the  depth  of  from  6,000  to  10,000  feet,  or 
even  more.  These  variations  in  the  depth  will  have  been 
accompanied  by  changes  in  the  temperature  of  the  water, 
in  the  supplies  of  air  and  food,  and  in  the  nature  of  the 
enemies  from  which  those  living  on  the  sea-floor  had  to 
defend  themselves. 

The  slow  evolution  of  animals  during  the  time  of  the 
Chalk,  and  in  many  similar  cases,  presents  exactly  the 
progress  required  by  Darwin's  theory.  In  other  cases  the 
quick  changes  of  animals  have  been  clearly  due  to  alter- 
ations in  the  conditions  of  life.  For  example,  the  evolution 
of  the  pigmy  elephants  of  Malta  (see  p.  279),  and  the  corre- 
sponding race  of  pigmy  hippopotami  that  lived  in  Crete  and 
Cyprus,  was  clearly  due  to  the  dwindling  islands  yielding 
diminished  supplies  of  food.  The  larger  animals  died  off; 
the  smallest  individuals  were  fittest  for  the  changed  con- 
ditions ;  and  the  Maltese  elephants  were  quickly  reduced 
to  a  race  of  pigmies.  The  origin  of  these  dwarf  elephants 
was  obviously  due  to  the  external  surroundings,  and  not 
to  an  innate  impulse. 

The  control  of  external  influences  is  repeatedly  shown 
when  animals  belonging  to  different  groups  acquired  the 
same  shape.  Thus  some  fish,  some  sea-living  reptiles, 
and  some  marine  mammals,  such  as  the  dolphins,  have  all 
acquired  somewhat  the  form  of  our  naval  torpedo.  They 
are  fish-like,  and  are  blunt  in  front  and  taper  towards  the 
tail.  It  seems  improbable  that  the  ancestors  of  various 
species  of  fish,  of  the  reptile  Ichthyosaurus,  and  of  the 
dolphins,  should  at  different  times  in  the  earth's  history 
have  become  possessed  by  an  internal  impulse  to  acquire 

287 


The  Driving  Power  of  Evolution 

the  form  of  a  torpedo;  but  these  different  animals  took 
to  life  in  the  sea,  and  the  shape  of  all  of  them  was  deter- 
mined by  the  same  surprising  mechanical  fact.  At  first 
sight  it  would  appear  easier  to  tow  a  tapering  log  of  wood 
point  foremost ;  but  every  boatman  knows  that  it  is  easier 
to  tow  such  an  object  with  the  blunt  end  in  front.  The 
superiority  of  this  position  is  not  due  to  an  innate  impulse 
in  the  spar  of  wood  to  behave  better  when  travelling  butt 
end  forward  and  the  point  astern.  The  behaviour  of  a 
towed  log  and  the  acquisition  of  the  fish-shaped  form  by 
various  marine  animals  are  both  due  to  obedience  to  the 
mechanical  forces  which  control  the  formation  of  eddies 
in  water. 

The  camel-like  form  of  the  extinct  Macrauchenia, 
which  lived  on  the  Pampas  of  South  America,  was  prob- 
ably acquired  owing  to  the  animal  having  adopted  similar 
habits  to  those  of  the  camel  (Gr.  makros,  long;  auchen,  neck). 

Again,  the  development  of  groups  of  carnivorous 
animals,  whenever  there  were  herbivorous  creatures  on 
which  to  prey,  is  more  easily  derived  from  the  opportunity 
afforded  by  the  abundant  food  than  by  an  internal  impulse. 

It  is  doubtless  often  difficult  and  sometimes  impossible 
to  explain  why  at  certain  periods  in  the  earth's  history 
there  was  a  sudden  change  in  the  life  of  the  earth.  The 
explanation,  that  the  higher  forms  succeeded  lower  forms 
in  consequence  of  the  survivors  having  beaten  their  pre- 
decessors in  the  struggle  for  existence,  is  sometimes 
unsatisfactory,  because  the  two  groups  may  not,  in  point 
of  time,  have  actually  overlapped.  The  later  forms  may 
have  rapidly  increased  in  number  and  variety  owing  to 
freedom  from  competition,  in  consequence  of  the  dis- 
appearance of  their  predecessors.  It  has  often  been 
suggested,  for  example,  that  the  disappearance  of  the 
great  reptiles  and  the  sudden  development  of  mammals  at 
the  end  of  the  middle  period  of  the  earth's  history  were 

288 


MACRAUCHENIA  PATACHONICA 

The  fossil  remains  of  this  animal  are  found  in  the  Pampas  of  the  Argentina.     It  had 
three-toed  feet,  was  as  large  as  a  camel,  and  had  a  short  trunk. 


The  Driving  Power  of  Evolution 

due  to  the  reptiles  having  been  exterminated  by  the  more 
intelligent  race  of  mammals ;  but  the  giant  reptiles  may 
have  become  extinct,  and  thus  left  the  field  free  for  the 
mammals. 

The  Middle  Period  in  the  history  of  the  world  came 
to  an  end  after  the  deposition  of  the  last  of  the  British 
Chalk ;  and  then  with  dramatic  suddenness  came  a  remark- 
able change  in  animal  life.  Great  reptiles  still  lived  on  in 
a  few  localities,  though  in  small  numbers;  and  the 
mammals,  which  had  only  managed  to  survive  by  having 
remained  small  and  inconspicuous,  now  took  the  place 
of  the  reptiles,  whom  they  had  hitherto  been  obliged  to 
evade. 

Whether  the  mammals  displaced  the  reptiles  or  whether 
they  replaced  them  we  do  not  know.  The  mammals,  owing 
to  their  greater  intelligence,  may  have  destroyed  the 
reptiles  by  preying  on  their  eggs  and  young,  or  by  con- 
suming their  food,  or  by  constantly  harassing  them.  As 
many  of  the  larger  reptiles  on  land  disappeared  before  the 
mammals  had  begun  to  increase  in  numbers,  and  as  the 
gigantic  marine  reptiles  became  extinct  at  about  the  same 
time  and  apparently  before  any  mammals  had  begun  to 
live  in  the  sea,  it  is  possible  that  the  mammals  were  not 
responsible  for  the  extermination  of  the  great  saurians. 
It  appears,  rather,  that  the  mammals  took  advantage  of 
the  changed  conditions,  and  replenished  the  earth  after 
the  saurians  had  gone,  than  that  they  defeated  and 
destroyed  them  in  the  struggle  for  existence. 

A  climatic  change  might  explain  the  end  of  the  reign  of 
the  reptiles.  If  the  climate  of  the  world  had  become 
colder,  the  reptiles  would  have  been  destroyed,  for  they 
have  not  the  power  of  birds  and  mammals  to  keep  the 
body  at  a  uniform  temperature,  in  spite  of  changes  in  the 
temperature  of  their  surroundings. 

Clocks  are  provided  with  an  arrangement  by  which  they 

289  T 


The  Driving  Power  of  Evolution 

move  at  the  same  rate  in  both  hot  and  cold  weather,  and 
mammals  have  a  mechanism  by  which  the  heat  of  the 
body  is  kept  nearly  the  same  in  spite  of  external  changes. 
It  is  owing  to  this  power  that  men  can  live  in  the  coldest 
of  polar  climates  and  in  the  hottest  regions  of  the  tropics, 
and  can  work  uninjured  in  ovens  at  temperatures  of 
400°  F.  A  reptile  has  no  such  control  over  its  body 
temperature.  Hence  British  snakes  are  only  able  to 
survive  the  winter  by  hiding  in  holes  where  they  are 
protected  from  the  cold.  They  are  killed  by  exposure  to 
severe  frost  or  great  heat.  I  once  kept  a  snake  on  a 
patch  of  dry  sand  on  a  hot  midsummer  day  in  central 
Australia,  and  in  five  minutes  the  heat  had  killed  it. 

If  the  climate  of  the  world  had  become  colder  at  the 
end  of  the  Chalk  Period,  that  change  might  have  destroyed 
the  reptiles  ;  and  this  explanation  is  supported  by  some 
indications  that  the  climate  became  colder  at  that  time. 
A  boulder  of  granite  was  found  in  the  chalk  at  Croydon, 
near  London;  and  this  boulder  was  probably  dropped 
from  an  iceberg  which  had  floated  southward  from  a 
northern  land.  There  is,  however,  no  adequate  evidence 
at  the  end  of  the  Chalk  Period  of  any  important  wide- 
spread climatic  change,  which  is,  indeed,  apparently  con- 
tradicted by  the  land  plants.  For  while  the  animal  life  of 
the  world  was  undergoing  a  revolution,  the  plants  remained 
comparatively  unaffected.  The  flora  of  the  end  of  the 
Era  of  Reptiles  lived  on  into  the  Era  of  Mammals.  This 
fact  tends  to  discredit  the  view  that  the  change  in  the 
animals  was  due  to  a  climatic  cause. 

A  climatic  change  would  not,  moreover,  explain  the 
extermination,  about  the  same  date,  of  many  animals  that 
lived  in  the  sea ;  for  many  of  them  lived  in  water  that 
would  be  barely  affected  by  any  change  in  climate,  though 
they  might  have  been  destroyed  indirectly  by  the  destruc- 
tion of  the  minute  organisms  which  formed  their  food. 

290 


The  Driving  Power  of  Evolution 

The  change  in  the  life  of  the  world  at  the  end  of  the 
Chalk  affected  animals  of  many  different  kinds  and  living 
under  all  sorts  of  conditions.  The  giant  land  saurians 
become  extinct  in  Europe  and  scarce  in  America.  The 
great  reptiles  of  the  sea  also  disappeared.  The  shellfish 
known  as  Ammonites,  which  were  allies  of  the  Nautilus, 
after  attaining  their  largest  size  in  the  Chalk,  were  exter- 
minated. The  Belemnites,  among  the  commonest  fossils 
in  the  clays  of  the  Era  of  Reptiles,  were  thought  to  have 
become  extinct  at  the  same  time  as  their  cousins  the 
Ammonites;  but  recently  specimens  of  them  have  been 
found  in  slightly  later  beds  in  western  Asia.  The  tri- 
angular two-valved  shellfish,  the  Trigonias,  disappeared 
from  European  seas  and  have  survived  only  in  those 
around  Australia.  The  typical  sea-urchins  and  starfish 
of  the  Chalk,  and  the  flat-toothed  sharks  that  preyed  on 
them,  died  about  the  same  time. 

There  is  no  satisfactory  explanation  of  this  widespread 
destruction.  It  is  perhaps  safest  to  admit  our  inability  to 
determine  its  real  cause.  But  it  does  suggest  that  there 
was  then  a  sudden  spurt  in  evolution  after  a  long  period  of 
slow  change  or  rest.  Some  groups  of  animals  may  then 
have  rapidly  developed  into  higher  types,  and  those  which 
did  not  join  in  this  movement  were  beaten  in  the  struggle 
for  existence,  and  became  extinct  or  lingered  on  in  sheltered 
localities. 

It  is,  however,  important  to  notice  that  each  of  the 
great  and  sudden  changes  in  the  animal  life  of  the  globe 
happened  at  periods  of  great  geographical  change,  when 
mountain-chains  were  being  uplifted,  sea  and  land  were 
changing  place,  and  volcanoes  were  in  especially  active 
eruption. 

These  changes  must  have  been  attended  by  great 
instability  in  the  weather.  The  climate — that  is,  the 
average  of  the  weather — may  have  remained  essentially 

291 


The  Driving  Power  of  Evolution 

the  same  for  the  world  as  a  whole  ;  but  the  weather  itself 
may  have  been  unusually  boisterous  and  fickle.  The 
changing  positions  of  land  and  sea  would  have  affected 
rainfall,  diverted  ocean  currents,  and  modified  the  dis- 
tribution of  temperature ;  the  tilting  and  uplift  of  the 
land  would  have  changed  the  courses  of  the  rivers  ;  some 
of  them  would  have  been  drained,  and  others  turned  into 
lakes ;  and  the  lakes  may  have  been  suddenly  discharged 
so  as  to  have  flooded  the  plains  beside  the  rivers,  and 
have  overwhelmed  the  animals  that  lived  on  them  in 
catastrophic  deluges. 

Many  of  the  animals  which  escaped  death  by  sudden 
geographic  accidents  might  have  died  in  consequence  of 
the  irregularity  of  the  seasons ;  this  would  also  have 
reduced  their  food-supply,  and  upset  their  regular  course 
of  life,  and  have  thus  rendered  them  sterile.  It  is  well 
known  that  even  the  very  primitive  organisms  which  give 
rise  to  infectious  diseases  come  and  go  for  no  obvious 
reason.  The  organism  which  produced  plague  decimated 
Europe  periodically  for  centuries ;  the  last  great  epidemic 
in  England  was  that  of  1665.  The  disease  has  often  been 
reintroduced,  but  for  some  reason  it  has  never  spread  to 
any  serious  extent.  Many  attempts  have  been  made  to 
explain  its  disappearance  as  the  result  of  improved  sani- 
tation and  of  other  changes;  but  those  explanations  do 
not  agree  with  the  facts.  According  to  Professor  C.  J. 
Martin,  "  the  plague  has  disappeared  from  Western 
Europe  for  reasons  which  we  do  not  understand  in  the 
very  least."  As  we  do  not  understand  the  disappearance 
of  this  organism  in  modern  times  from  our  own  country, 
it  is  not  surprising  that  the  disappearance  of  the  reptiles 
of  the  Mesozoic  Era  is  also  unexplained. 

Animals  which  are  kept  in  captivity  often  lose  the 
power  of  producing  offspring,  and  a  similar  change  happens 
from  slight  interruptions  in  their  ordinary  course  of  life. 

292 


The  Driving  Power  of  Evolution 

Perhaps  the  most  probable  explanation  of  the  simul- 
taneous extinction  of  many  different  kinds  of  animals  is 
that  their  rate  of  breeding  was  reduced  by  slight  changes 
in  climate  and  food-supply  that  occurred  at  periods  of 
great  geographic  change. 

Plants  would  be  less  influenced  by  such  changes. 
Hence  the  vegetable  food-supply  would  have  remained 
unchanged,  and  new  animals  would  have  been  rapidly 
developed  to  take  the  place  of  those  which  had  perished. 

It  would  therefore  appear  that  the  great  break  between 
the  life  of  the  middle  period  of  the  earth  and  of  the 
existing  period  was  rather  due  to  the  influence  of  wide- 
spread geographical  changes  than  to  an  internal  tendency 
in  the  animals  to  change.  The  evolution  of  modern  life 
from  that  of  earlier  eras  was  probably  stimulated  by 
external  conditions  and  not  by  internal  influences,  for 
each  of  the  chief  changes  in  the  animal  life  of  the  earth 
occurred  at  times  of  especially  great  geographic  change. 


293 


CHAPTER  XXI 
THE  SIZE  OF  EXTINCT  ANIMALS 

THE  size  and  shapes  of  animals  have  always  been  limited 
by  the  same  mechanical  conditions  as  control  them  to-day. 
The  force  of  gravity  appears  to  have  been  practically 
invariable  on  the  earth's  surface  so  long  as  large  animals 
have  lived  upon  it.  Hence  the  extinct  animals  have 
been  subject  to  the  same  limitations  in  weight  as  their 
modern  descendants.  Prof.  Barr  has  called  attention  to  the 
mechanical  principle  which  prevents  any  active  quadruped 
living  on  land  from  acquiring  a  much  heavier  body  than 
the  modern  elephant;  for  its  legs  can  only  support  the 
weight  of  the  body  as  they  are  straight  and  thick,  and 
are  placed  vertically  in  the  best  possible  manner  for 
supporting  a  heavy  mass.  Animals  with  bent  legs  like  a 
cat  have  light  bodies,  and  those  with  long,  straggling  legs 
like  a  spider  have  extremely  small  bodies.  Shakespeare 
expressed  these  principles  when  he  makes  Ulysses  say: 
"  The  elephant  hath  joints,  but  none  for  courtesy :  his 
legs  are  legs  for  necessity,  not  for  flexure." 

The  weight  of  the  body  which  the  legs  can  support  is 
controlled  by  the  rule  which  Prof.  Barr  states  as  follows : 
"  The  weights  of  similar  structures  therefore  increase, 
with  increase  of  dimensions,  in  a  higher  ratio  than  the 
loads  which  the  structures  are  alike  suitable  to  bear."1 

He  illustrates  this  principle  by  comparison  between 
the  Britannia  and  Forth  Bridges.  In  the  Britannia 

294 


The  Size  of  Extinct  Animals 

Bridge  the  length  of  the  span  is  460  feet,  and  that  of  the 
Forth  Bridge  is  1,710  feet.  The  smaller  span  of  the 
Britannia  Bridge  is  maintained  by  a  girder,  of  which  the 
greatest  depth  is  30  feet.  In  order  to  support  the  span 
of  the  Forth  Bridge  the  girder  (for  the  cantilever  is  a 
girder  supported  in  the  middle)  has  been  built  with  the 
depth  of  350  feet.  The  span  of  the  Forth  Bridge  is 
37  times  that  of  the  Britannia  Bridge,  but  it  requires  a 
girder  11-4  times  as  deep.  The  structure  of  animals  is 
governed  by  the  same  requirements.  If  the  weight  of  the 
body  and  its  length  are  increased,  the  supports  have  to  be 
increased  in  a  much  greater  proportion.  According  to 
Prof.  Barr's  calculations,  animals  with  a  body  two  or  three 
times  as  long,  broad,  and  thick  as  the  elephant  "  would 
require  legs  nearly  filling  up  the  whole  space  under  its 
body."2  It  may  appear  at  first  sight  that  dimensions 
which  have  been  quoted  for  some  extinct  monsters  con- 
tradict this  principle,  for  Diplodocus  is  far  more  massive 
than  the  elephant,  and  yet  has  only  four  legs.  Diplodocus, 
however,  was  amphibious,  so  that  the  weight  of  its  body 
was  generally  supported  by  water,  and  it  was  therefore 
free  from  the  restriction  placed  on  a  land  animal.  Appar- 
ently no  active  land  animal  has  ever  been  appreciably 
more  massive  than  the  elephant.  The  view  that  many  of 
the  ancestors  of  modern  creatures  were  larger  than  their 
existing  descendants  is  inconsistent  with  the  facts.  The 
reptiles  were  no  doubt  larger  when  they  were  the  dominant 
group ;  but  after  they  had  been  beaten  in  the  struggle  for 
existence,  they  diminished  in  size,  and  the  mammals 
surpassed  them. 

The  earliest  Amphibians  were  much  larger  than  any 
living  members  of  that  class ;  but  they  were  smaller  than 
living  animals  with  the  same  habits  of  life,  and  they  were 
probably  only  from  one-third  to  one-fourth  the  length  of 
the  reptiles  in  the  next  geological  era.  Again,  in  most  of 

295 


The  Size  of  Extinct  Animals 

the  chief  groups  of  mammals  the  living  animals  are 
generally  larger  than  their  ancestors.  Thus  the  horse, 
camel,  giraffe,  rhinoceros,  hippopotamus,  and  elk  are  all 
larger  or  as  large  as  their  predecessors ;  and  it  is  only  in 
the  smaller  groups  in  which  the  extinct  forms  were  by  far 
the  largest.  There  is  no  known  rat  as  large  as  that  which 
once  lived  on  the  island  of  Anguilla  in  the  West  Indies, 
but  it  was  smaller  than  many  existing  land  animals.  In  a 
few  groups,  such  as  the  marsupials  and  the  South  American 
giant  sloths  and  armadilloes,  extinct  forms  were  the 
largest ;  but  they  only  became  extinct  in  very  recent 
geological  times,  and  they  were  not  larger  than  some 
living  animals. 

"  It  is  a  great  mistake  to  conclude,"  says  Sir  Ray 
Lankester,  "  that  it  is  a  law  of  Nature  that  recent  animals 
are  all  small  and  insignificant  as  compared  with  their 
representatives  in  the  past.  That  is  simply  not  true. 
Recent  horses  are  bigger  than  extinct  ones,  and  much 
bigger  than  the  three-toed  and  four-toed  ancestors  of 
horses.  Recent  elephants  are  as  big  as  any  that  have 
existed,  and  much  bigger  than  the  earlier  elephantine 
ancestors.  There  never  has  been  any  creature  of  any 
kind — mammal,  reptile,  bird,  or  fish — in  any  geological 
period  we  know  of,  so  big  as  some  of  the  existing  whales 
— the  Sperm  Whale,  the  Great  Rorqual,  and  the  Whale- 
bone Whales.  It  is  true  that  there  were  enormous  reptiles 
in  the  past — far  larger  than  any  living  crocodiles,  standing 
fourteen  feet  at  the  loins  and  measuring  eighty  feet  from 
the  tip  of  the  snout  to  the  tip  of  the  tail ;  but  their  bodies 
did  not  weigh  much  more  than  that  of  a  big  African 
elephant,  and  were  small  compared  with  whales."3 

The  mechanical  conditions  that  control  the  size  of  flying 
animals  have  prevented  flying  birds  ever  developing  heavy 
bodies.  Dragon  flies  have  been  found  with  wings  two 
feet  across,  and  the  flying  reptile,  the  Pteranodon,  had  an 

296 


THE  PTERANODON,  THE  GREAT  GLIDING  REPTILE  FOUND  IN  KANSAS 

The  skull  is  2\  feet  long  and  the  expanse  of  wing  about  18   feet.     It  is  represented 
crawling  up  the  cliff,  whence  it  will  glide  downward  in  order  to  swoop  upon  its  prey. 


The  Size  of  Extinct  Animals 

expanse  of  wing  far  greater  than  that  of  any  modern 
flying  animal.  Nevertheless,  it  is  doubtful  if  any  creature 
which  had  the  power  of  real  flight  has  been  larger  than 
the  modern  albatross.  The  Pteranodon  and  other  reptiles 
that  lived  in  the  air  probably  had  only  the  power  of  gliding 
from  tree  to  tree  like  the  flying  squirrels  (PL  XVIII.) ; 
their  wings  could  not  have  been  used  for  flight  by  the 
rapid  beating  of  the  air,  for  the  wing  membrane  was  in- 
adequately supported ;  and  those  reptiles  which  could  fly 
as  well  as  the  bat  were  all  very  small  animals. 

The  size  of  birds  is  governed  by  the  fact  that,  as 
Prof.  Barr  has  pointed  out,  the  weight  of  an  engine 
increases  much  more  rapidly  than  its  power.  In  similar 
engines  the  powers  will  increase  as  the  squares  and  the 
weights  as  the  cubes  of  their  linear  dimensions.  Much 
more  power  can  therefore  be  gained  from  a  given  weight 
of  material  by  using  it  in  several  small  engines  than 
in  one  big  engine.  Hence,  flying  machines  and  motor 
cars  are  provided  with  many  cylinders,  which  are  practic- 
ally separate  engines,  instead  of  with  one  large  engine. 
The  body  of  a  bird  consists  of  one  engine ;  so  its  weight 
increases  far  more  rapidly  than  its  power.  There- 
fore, as  birds  have  grown  big  they  have  lost  the  power 
of  flight,  and,  like  the  ostriches,  emus,  and  moas,  only  run 
on  the  ground.  Some  large  bones  and  skulls  of  extinct 
birds  have  been  regarded  as  evidence  of  the  former 
existence  of  flying  birds  much  greater  than  any  which  live 
to-day ;  but  they  probably  only  ran  about  on  the  ground, 
for  when  the  structure  of  the  fore  limb  has  been  discovered, 
it  has  always  proved  to  be  so  small  that  the  bird  could  not 
have  flown. 

The  body  of  a  centipede  or  a  worm  may  be  regarded  as 
composed  of  many  engines ;  for  each  section  of  the  body 
contains  a  set  of  organs,  which  though  all  connected,  like 
the  cylinders  of  a  motor-car  engine,  have  still  much 

297 


The  Size  of  Extinct  Animals 

independence.  Hence,  if  a  group  of  centipedes  had  taken 
to  flying,  and  each  section  of  the  body  had  developed  a  pair 
of  wings  which  could  have  been  worked  by  an  independent 
set  of  muscles  and  guided  by  its  own  nerve  centre,  then, 
as  far  as  concerns  the  ratio  of  weight  to  power,  the  centi- 
pede might  have  acquired  gigantic  proportions.  This 
growth  has  possibly  only  been  prevented  by  the  difficulty 
of  obtaining  enough  fuel  in  the  form  of  food ;  but  for  this 
limitation  the  air  might  have  been  the  home  of  colossal 
flying  creatures,  which  might  have  rendered  life  on  the 
open  plains  possible  only  to  burrowing  animals. 

Trees  also  illustrate  the  control  in  size  exercised  by  the 
laws  of  similar  structures.  No  trees  are  known  from  any 
previous  period  which  have  been  larger  than  those  of  the 
present  day.  The  highest  living  trees  are  the  giant  gums 
of  south-eastern  Australia,  and  the  pines  of  the  Sierra 
Nevada  in  the  United  States.  The  Australian  gums  were 
somewhat  the  taller,  and  some  of  the  trees  in  both  groups 
were  over  400  feet  high.  The  claim  that  some  of  the 
Australian  gums  exceed  500  feet  in  height  does  not  appear 
to  have  been  established.  The  gums  and  sequoias  belong 
to  very  different  orders  of  plants.  The  gums  are  eucalypts 
and  the  great  trees  of  western  America  are  conifers ;  and 
the  fact  that  such  unrelated  trees  reach  approximately  the 
same  size  shows  that  the  limits  of  growth  are  fixed  by  the 
force  of  the  winds  acting  on  materials  of  similar  strength. 
The  trees  could  not  double  their  height  unless  they  could 
develop  timber  many  times  stronger  than  that  grown  by 
present  trees.  So  far  as  we  know,  modern  trees  are  larger 
than  any  of  their  predecessors.  Some  of  the  trees  of  the 
coal  measures  belonged  to  the  same  groups  as  the  living 
club-mosses  and  horsetails,  and  were  far  larger  than  any 
living  representatives  of  their  groups  ;  but  the  largest  of  the 
Carboniferous  trees  were  apparently  much  smaller  than 
the  largest  modern  trees.  Our  knowledge  of  fossil  trees  is 

298 


The  Size  of  Extinct  Animals 

so  fragmentary  that  it  would  be  unsafe  to  say  that  none 
has  ever  been  larger  than  those  now  living ;  but  there  is 
no  evidence  that  any  extinct  tree  ever  grew  to  the  same 
height  as  the  forest  giants  of  the  present  day. 

1  "  Comparisons  of  Similar  Structures  and  Machines,"  Trans.  Inst. 
Engineers  and  Shipbuilders,  Scotland,  1899,  vol.  xlii.,  p.  332. 

2  Ibid.,  p.  336. 

3  E.  Ray  Lankester,  "  Extinct  Animals,"  1905,  pp.  166,  167. 


299 


CHAPTER  XXII 
THE  GEOLOGICAL   HISTORY   OF   MAN 

THE  geological  history  of  man  is  naturally  one  of  the 
most  widely  interesting  of  all  branches  of  geology.  The 
materials,  however,  are  deplorably  scanty,  for  owing  to 
the  fragile  nature  of  human  bones,  to  the  habit  of  burial 
in  soils  where  bones  will  soon  decay,  to  the  unfortunate 
fact  that  from  a  lack  of  boats  primeval  man  was  seldom 
drowned  and  his  skeleton  was  rarely  preserved  in  the  mud 
floor  of  quiet  waters,  owing  to  cremation  and  cannibalism 
on  land,  the  fossil  remains  of  man  are  deplorably 
exceptional. 

The  search  for  human  fossils  has  been  eagerly  pursued, 
and  various  remains  have  been  regarded  as  human  on  no 
adequate  evidence.  Thus  a  Swiss  fossil  was  described  in 
the  eighteenth  century  as  the  Man  of  the  Deluge,  but 
has  proved  to  be  only  the  fragments  of  a  gigantic  newt- 
like  animal  (p.  247).  From  the  scarcity  of  fossil  human 
bones  the  geological  antiquity  of  man  was  first  clearly 
shown  by  his  stone  implements.  Monkeys  carry  about 
with  them  stones  with  which  they  break  nuts;  and  the 
men  who  lived  in  areas  with  abundant  pebbles  naturally 
used  them  as  their  first  weapons  and  tools.  The  discovery 
of  America  and  the  South  Sea  Islands  revealed  tribes  of 
men  who  to  that  day  used  stone  tools,  as  they  had  no 
metals.  Stone  tools  similar  to  those  used  by  modern 
men,  though  more  primitive  in  their  workmanship,  were 
subsequently  discovered  in  Europe.  Thus  one  was  found 

300 


The  Geological  History  of  Man 

at  the  end  of  the  seventeenth  century  in  London  in  Gray's 
Inn  Lane,  and  is  preserved  in  the  British  Museum.  There 
was,  however,  no  direct  evidence,  except  its  association 
with  some  elephant  bones,  to  show  that  this  stone  weapon 
was  of  any  great  antiquity.  That  the  use  of  stone  tools 
in  Europe  dates  far  back  into  prehistoric  times  was  first 
clearly  shown,  for  in  1849  they  were  found  in  the  gravels 
of  the  Somme  Valley  in  northern  France  by  Boucher  de 
Perthes.  His  discoveries,  however,  were  for  a  while 
discredited.  He  claimed  too  much.  The  flints  found  in 
gravels  are  extremely  varied  in  shape,  and  by  diligent 


FIG.  27.— Two  OF  DE  PERTHES'  ILLUSTRATIONS  OF  FIGURE  STONES, 
FROM  THE  PALEOLITHIC  GRAVELS  OF  THE  SOMME  VALLEY, 
NORTH  FRANCE. 

They  are  generally  regarded  as  naturally  shaped  stones,  with  an 
external  resemblance  to  natural  objects. 

collecting  from  a  bed  of  gravel  specimens  can  be  found 
resembling  in  form  birds,  beasts,  and  fish.  In  1849  De 
Perthes  published  a  great  work,  "  Antiquites  Celtiques  et 
Ante"diluviennes,"  containing  1,600  figures  of  the  flints 
from  the  Somme  Valley,  and  some  of  them,  such  as  those 
in  Fig.  27,  he  regarded  as  having  been  partly  shaped  by 
man  into  the  figures  of  animals.  Most  of  De  Perthes' 
contemporaries  justly  regarded  the  resemblances  as 
fanciful,  and  therefore  for  many  years  the  whole  of  his 
work  was  rejected.  The  late  Sir  Joseph  Prestwich  and 
Sir  John  Evans  visited  Abbeville,  and  separated  the  stones 
having  accidental  resemblances  to  natural  objects  from 

301 


The  Geological  History  of  Man 

those  which  had  been  chipped  by  man  for  use  as  imple- 
ments, and  these  implements  were  soon  universally  recog- 
nized as  artificial,  while  the  geological  evidence  was  con- 
clusive that  they  must  have  been  formed  by  man  at  least 
tens  of  thousands  of  years  ago. 

The  enormous  antiquity  of  man  has  also  been  shown  by 
the  discovery  of  his  implements  on  the  floors  of  some 
English  caves,  such  as  Kent  Cave  and  Brixham  Cave 
near  Torquay.  Remains  left  by  man  were  there  found 
buried  under  beds  of  limestone,  which  were  slowly  formed 
by  the  drip  of  water  from  the  roof  of  the  cave.  This 
water  contained  dissolved  material  which  was  deposited 
as  limestone  on  the  floor,  and  the  limestone  was  produced 
so  slowly  that  it  must  have  taken  many  millenniums  to 
form  the  layer  that  covered  the  original  resting-place  of 
these  relics  of  man.  Further,  it  is  clear  that  the  local 
valleys  have  been  considerably  deepened  by  natural  means 
since  the  caves  were  used  as  human  dwellings. 

The  date  of  the  earliest  Palaeolithic  man  cannot  be 
expressed  in  years.  Some  authorities  represent  him  as 
having  lived  fifty  thousand  years  ago,  while  others  estimate 
the  date  as  a  million,  and  others  at  perhaps  several  million 
years  ago.  But  the  data  are  so  uncertain  that  it  is  still 
safest  to  describe  the  different  ages  of  man  by  reference 
to  his  gradual  advance  in  culture,  without  attempting  to 
assign  definite  dates  in  years  to  any,  except  perhaps  to 
the  most  recent  of  these  ages. 

Further  discoveries  have  shown  that  the  men  who 
made  and  used  stone  implements  were  once  widespread 
in  western  Europe  and  in  England,  and  it  was  found  that 
their  implements  offer  two  chief  types.  Some  of  them 
were  only  chipped,  and  were  therefore  more  primitive  in 
their  character  than  those  which  were  ground  to  a  smooth 
edge,  like  those  of  living  Australian  aborigines  and  West 
Indian  Caribs.  The  chipped  stones  were  often  found  in 

302 


The  Geological  History  of  Man 

gravels  high  above  the  levels  of  the  present  streams,  and 
were  therefore  geologically  older  than  the  ground-edged 
implements  which  were  often  found  nearer  the  levels  of 
the  existing  rivers.  The  chipped  implements  were  there- 
fore called  by  Sir  John  Lubbock,  afterwards  Lord  Ave- 
bury,  Palaeolithic,  as  they  represent  "  the  old  stone  age," 
and  those  with  ground  edges  he  called  Neolithic,  as 
they  belong  to  the  "  new  stone  age." 

The  geological  history  of  man  is  divided  into  five  stages : 
(i)  Various  classes  of  evidence  from  before  the  date  of 
Palaeolithic  man;  (2)  the  Palaeolithic,  the  period  of  the 
old  stone  implements ;  (3)  the  Neolithic,  the  period  of  the 
new  stone  implements;  (4)  the  Bronze  Age,  when  man 
made  and  left  behind  tools  and  weapons  of  bronze ;  and 
(5)  the  Iron  Age,  from  which  iron  tools  have  also  been 
preserved. 

The  implements  made  by  man  are  more  often  found 
than  his  bones,  so  that  the  classification  based  on  his 
tools  is  the  more  convenient.  Pre-Palaeolithic  man  has 
been  called  Eolithic,  and  many  rudely  chipped  flints 
have  been  called  "  eoliths,"  and  attributed  to  the  work 
of  the  earliest  men. 

THE  ORIGIN  OF  EOLITHS. 

These  eoliths  were  discovered  by  Mr.  Benjamin  Harri- 
son, and  described  as  of  human  origin  by  the  late  Sir 
Joseph  Prestwich  in  1891  and  1892 ;  but  one  of  the 
shrewdest  of  British  archaeologists,  the  late  Sir  John 
Evans,  denied  their  human  origin  in  his  Presidential 
Address  to  the  British  Association  in  1897  >  and  the 
question  whether  they  are  natural  or  artificial  has  given 
rise  to  prolonged  controversy.  While,  in  1897,  acting 
as  guide  to  an  excursion  to  the  Eolithic  gravels  on  the 
Downs  above  Ightham,  we  were  met  by  Mr.  Harrison, 
who  kindly  invited  us  to  see  his  collection,  and  gave  me 

303 


The  Geological  History  of  Man 

several  specimens  in  addition  to  those  we  had  previously 
found.  Examination  of  his  specimens,  selected  out  of 
many  tens  of  thousands  of  chipped  flints,  and  Mr.  Harri- 
son's enthusiastic  faith,  persuaded  me  of  their  human 
workmanship.  Repeated  examination  of  the  specimens 
during  the  next  few  days,  however,  suggested  doubts, 
owing  to  the  fact  that  the  stones  had  been  chipped  at 
different  ages  and  from  the  same  side,  as  if  the  chipping 
were  due  to  the  thin,  brittle  edge  formed  where  two  frost 
flakes  met,  having  been  broken  away  by  movements  of 
the  gravel.  On  suggesting  this  hypothesis  to  Mr.  William 
Cunnington,  the  Wiltshire  archaeologist,  he  visited  Ight- 
ham,  and  was  also  at  first  convinced  of  the  human  shaping 
of  the  stones.  But  his  further  study  disclosed  strong 
evidence  against  the  artificial  chipping  of  the  stones.  The 
eoliths  have  been  formed  by  five  successive  processes. 
First,  flints  are  split  by  frost,  which  breaks  off  large 
curved  flakes,  and  the  depressions  formed  by  two  frost- 
flakes  may  meet  along  a  thin  edge.  Second,  a  thin, 
siliceous  film  was  deposited  on  these  flaked  surfaces,  and 
the  stone  stained  brown  by  iron.  Third,  the  flints  were 
marked  by  scratches  which  resemble  those  made  by  sand 
during  glacier  action.  Fourth,  a  second  coat  of  silica 
was  deposited,  filling  up  the  scratches.  Fifth,  many  of 
the  stones  were  polished  by  sand  during  intervals  of 
drought  as  they  lay  on  the  bed  of  the  river  which  formed 
the  gravels. 

During  these  processes  any  thin  edges  left  by  frost 
flaking  were  chipped,  probably  by  being  pressed  against 
other  flints  during  movements  caused  by  the  thawing  of 
the  frozen  gravel.  The  chipping  supposed  to  be  the  work 
of  Eolithic  men  was  clearly  done  at  three  different  times. 
Some  of  the  chips  were  made  before  the  flints  were 
stained,  a  second  set  after  the  staining  but  before  the 
so  ratching,  and  the  last  series  after  the  scratching.  Mr. 

304 


The  Geological  History  of  Man 

Cunnington  strengthened  his  case  by  remarking  the  use- 
lessness  of  many  of  the  supposed  implements.  Those  who 
believe  in  the  human  manufacture  of  the  Eoliths  suggest 
that  the  concave  edge,  which  is  produced  where  two 
hollows  meet,  was  used  for  shaping  and  smoothing  sticks ; 
and  those  stones  with  a  single  point  projecting  between 
two  concavities  were  used  as  borers.  The  stones  would 
have  been  very  inefficient  for  either  purpose,  while  the 
chipping  sometimes  spoiled  edges  which  would  have  been 
originally  sharp  and  smooth,  and  would  have  cut  much 
better  than  after  they  were  chipped.  Mr.  Cunnington 
concluded  in  his  paper  of  1897 :  "  And  as  the  chipping  of 
the  flints  was  apparently  caused  by  the  action  of  extreme 
cold  upon  the  gravels,  movements  in  which  were  produced 
by  the  action  of  ice,  I  propose  for  these  shaped  flints  the 
name  of  '  Glacioliths."'1 

Objection  to  this  explanation  of  the  natural  origin  of 
the  Eoliths  was  taken  on  the  grounds  that  Palaeolithic 
implements  had  also  been  found  on  this  plateau,  so  that 
man  must  have  been  in  existence  at  the  time ;  but  these 
Palaeolithic  implements  have  themselves  been  eolithically 
chipped.  One  interesting  specimen  was  described  and 
figured  by  Mr.  Cunnington,  in  which  a  true  Palaeolithic 
implement  had  been  broken  across,  probably  by  frost, 
and  the  broken  edge  had  then  been  eolithically  chipped. 
The  occurrence  of  these  chipped  stone  implements 
shows  that  if  the  Eolithic  chipping  were  artificial, 
Eolithic  man  lived  after  his  supposed  Palaeolithic  suc- 
cessor. 

These  views  as  to  the  natural  formation  of  the  Eoliths 
by  a  soil-cap  movement  received  no  support  for  several 
years,  until  Prof.  Marcellin  Boule  noticed  the  resemblance 
to  Eolithic  chipping  of  flints  which  had  been  rubbed 
against  one  another  in  chalk  mills ;  and  since  that  time 
the  view  that  the  Eolithic  chipping  is  due  to  soil-cap 

305  u 


The  Geological  History  of  Man 

movements  has  been  widely  accepted,  though  many  high 
authorities  still  accept  the  Eoliths  as  made  by  primitive 
man. 

THE  SUB-CRAG  IMPLEMENTS. 

Weightier  evidence  of  the  antiquity  of  man,  greater 
even  than  that  claimed  from  the  Ightham  "  Eoliths," 
has  been  produced  from  the  deposits  in  Suffolk  known 
as  "  the  Crags."  The  Crags  are  divided  into  three  divi- 
sions. The  earliest  member  of  this  series  is  the  Coralline 
Crag ;  it  is  a  deposit  rich  in  shells,  which  indicate  that 
the  British  seas  were  much  warmer  than  they  are  at 
present.  The  second  division  consists  of  the  Red  Crag  ; 
the  third  division  is  the  Norwich  Crag.  In  gravels  at  the 
base  of  both  the  Red2  and  Norwich  Crags  chipped  flints 
have  been  found  which  are  regarded  as  of  human  work- 
manship. These  worked  flints  are  very  different  in 
character  from  the  ordinary  Palaeolithic  stone  implements. 
They  are  beak-shaped,  and  are  therefore  called  the  rostro- 
carinate  implements.  They  have  been  described  in  detail 
by  Sir  Ray  Lankester,  who  claims  that  the  evidence  for 
their  human  workmanship  is  unanswerable,  and  many 
eminent  authorities  on  stone  implements  accept  them 
as  undoubtedly  made  by  man.  After  a  careful  examination 
of  some  of  the  specimens  the  evidence  for  their  human 
origin  seems  to  me  conclusive. 

Their  geological  position  indicates  that  they  are  far 
older  than  any  implements  yet  well  established,  and  they 
may  be  coeval  with  the  monkey-man  of  Java.  Their 
antiquity,  however,  may  not  be  so  great  as  the  usually 
accepted  interpretation  of  the  Crags  would  at  first  suggest  ; 
for  their  discovery  below  the  Red  Crag  would  appear  to 
carry  them  back  to  the  Middle  Pliocene,  and  to  a  period 
in  which  the  British  climate  was  much  warmer  than  at 
present.  Sir  Ray  Lankester  has  pointed  out  that  most  of 

306 


The  Geological  History  of  Man 

the  Red  Crag  consists  of  a  series  of  redeposited  beds,  and 
the  shells  that  indicate  warm  conditions  are  only  frag- 
ments derived  from  the  older  Coralline  Crag.  The  un- 
weathered  shells  found  in  the  Red  Crag  show  that  the 
British  climate  was  colder  than  it  is  now,  and  that  it  had 
advanced  far  towards  the  cold  of  the  succeeding  Glacial 
Period.  The  Red  Crag  in  the  pit  south  of  Butley  in 
Suffolk  has  structures  characteristic  of  beds  formed  as 
sand-dunes ;  the  shells  in  it  are  wind-worn,  and  mainly 
occur  as  fragments,  and  in  the  shelly  sand  are  many 
vertical  pipes  rising  from  long  branching  horizontal  stems 
which  indicate  the  former  presence  of  the  creeping  under- 
ground stems  of  ordinary  dune  grasses.  Such  deposits 
may  accumulate  very  quickly,  and  justify  Mr.  Whitaker's 
remark  that  the  Red  Crag  "  probably  represented  only  a 
very  insignificant  period  of  time."3  The  Red  Crag  was 
probably  formed  long  after  the  Coralline  Crag,  and  during 
the  approach  of  that  cold  period  which  is  represented  in 
various  parts  of  the  Continent  by  a  Glacial  Period,  of 
which  there  is  no  direct  evidence  in  this  country.  Hence, 
though  the  sub-Crag  implements  prove  that  man  was  pre- 
glacial  in  this  country,  they  are  compatible  with  the 
evolution  of  man  at  the  very  end  of  the  Pliocene,  or  the 
beginning  of  the  Pleistocene. 

PALAEOLITHIC  MAN. 

Palaeolithic  is  the  name  of  a  stage  in  human  culture, 
and  not  of  a  particular  race.  It  describes  the  stage  in 
which  all  men  used  chipped  stone  implements,  and  were 
in  a  very  primitive  stage  of  civilization.  The  earlier 
Palaeolithic  men  lived  in  river  valleys,  where  they  must 
have  constructed  rude  shelters  of  wood  and  skins.  Later 
Palaeolithic  men  lived  in  caves  and  rock  shelters.  They 
were  all  hunters  and  fishermen ;  they  lived  on  food  killed 

307 


The  Geological  History  of  Man 

in  the  chase  or  caught  by  fishing,  and  on  shell- fish  and 
fruits  collected  on  the  shore  or  in  the  forest.  They 
had  no  knowledge  of  weaving,  and  must  have  been 
clad  in  skins ;  they  did  not  practise  agriculture,  and 
had  no  domestic  animals,  unless  perhaps  some  lines  on 
drawings  of  the  horse  and  reindeer  made  by  later  Palaeo- 
lithic man  were  intended  to  represent  harness.  Early 
Palaeolithic  man  apparently  used  no  pottery,  which  is  first 
found  amongst  the  relics  of  later  Palaeolithic  man. 

The  Palaeolithic  stage  is  divided  into  a  series  of  sections, 
which  are  named  after  French  and  Belgian  localities. 
The  progress  from  section  to  section  shows  a  gradual 
improvement  in  the  stone  implements ;  while  the  bone 
implements,  carvings,  and  drawings  of  animals  made  by 
later  Palaeolithic  man  show  high  artistic  skill.  The  later 
Palaeolithic  man  had  probably  developed  a  simple  form  of 
religion,  since  his  drawings  were  of  animals  which  were 
useful  to  him,  such  as  the  horse,  reindeer,  mammoth, 
cattle  and  goat ;  the  lion,  hyena,  and  wolf,  which  would 
have  been  his  most  serious  enemies,  were  not  drawn.  It 
has,  therefore,  been  reasonably  suggested  that  the  draw- 
ings were  made  for  the  purposes  of  magic,  and  were 
intended  to  give  man  some  power  over  the  animals 
represented. 

The  different  sections  of  Palaeolithic  man  are  enumer- 
ated in  the  table  on  p.  309,  in  which  the  most  recent  are 
at  the  top. 

The  men  of  the  three  oldest  sections  lived  in  river  drifts. 
The  three  next,  from  the  Aurignacian  to  Magdalenian, 
were  cave-dwellers.  The  Azilian,  found  in  the  south-west 
of  France,  may  represent  a  passage  from  Palaeolithic  man 
to  his  Neolithic  successors.  The  stone  implements  of  the 
three  lower  stages  are  said  in  France  to  occur  in  distinct 
beds;  but,  according  to  Prof.  Boyd  Dawkins,  the  three 
types  occur  in  England  in  the  same  deposit. 

308 


The  Geological  History  of  Man 

The  bones  of  the  men  who  made  these  implements  are 
of  extreme  interest,  because  they  are  the  first  direct  evi- 
dence of  the  development  of  man.  The  two  oldest  fossils 
which  have  to  be  considered  in  this  connection  are  those 
of  the  skull-top  and  thigh  bone  which  were  found  forty-six 
feet  apart  in  a  bed  of  gravel  in  Java.  They  were  named 
by  their  discoverer,  Dubois,  Pithecanthropus — the  monkey- 
man  (Gr.  pithekos,  an  ape;  anthropos,  man). 


Dwellings. 

Human 
Remains. 

Charac- 
teristic 
Animals.* 

Climatic 
Conditions. 

C.  Azilian  :  Intermediate 

between    Palaeo- 

lithic and  Neolithic 

(  Magdalenian 
B.  Upper  -j  Solutrian 
lAurignacian 

Cave) 
do.    [ 
do.  K 

Modern 
types 

Reindeer, 
etc. 

Post  -  glacial 
cold  climate 

fMousterian 

River  1 

Neander- 

Mam- 

Glacial   con- 

valleys f 

thal  by 

moth* 

ditions    in 

A.  Lower     Acheulean 

do.    )} 

Diisseldorf 
Piltdown 

Red-  deer* 

Scotland 

\ 

(Sussex) 

IChellean 

do.       J 

Elephas 

Warmer    cli- 

antiquus* 

mate  after 

the  glacia- 

t  i  on      of 

England 

*  These  animals  were  not  restricted  to  the  horizons  mentioned. 

The  affinities  of  this  animal  have  been  greatly  discussed. 
It  appears  to  be  now  generally  agreed  that  it  was  a 
creature  intermediate  between  man  and  the  man-like  apes, 
and  that  it  was  more  closely  related  to  the  higher  monkeys, 
such  as  the  chimpanzee,  than  to  man.  The  top  of  the 
skull  is  much  flatter  than  that  of  man,  so  that  the  cavity 
for  the  brain  was  much  smaller,  and  corresponded  more 
to  that  of  a  chimpanzee  than  to  that  of  a  man.  Un- 
fortunately, the  monkey-man  is  only  known  from  part  of 

309 


The  Geological  History  of  Man 

the  skull,  and  a  leg-bone  that  may  have  belonged  to  the 
same  individual,  although  an  expedition  which  went  to 
Java  for  the  purpose  carried  out  extensive  excavations  in 
the  bed  of  gravel  from  which  the  fragments  had  been 
obtained.  The  age  is  generally  regarded  as  Pliocene 
from  the  associated  fossils,  but  it  is  at  present  impossible 
to  be  sure  of  the  relative  dates  of  beds  called  "  Upper 
Pliocene"  in  Java  and  those  of  Europe,  and  some 
authorities  regard  the  monkey-man  as  early  Pleistocene. 


FIG.  28. — COMPARATIVE  OUTLINES  OF  THE  TOP  OF  THE  SKULL. 

Gorilla.  Piltdown  man. 

—  .  —  .  —  Monkey-man  of  Java. Modern  man. 

Neanderthal  man. 

The  oldest  definitely  human  fossil  hitherto  discovered 
is  a  lower  jaw  found  in  1907  near  Heidelberg  in  Germany. 
It  is  so  different  from  the  jaw  of  modern  man  that  it  was 
recognized  as  that  of  a  distinct  species,  and  named 
Homo  heidelbergensis.  The  jaw  is  massive,  but  has  small 
teeth ;  it  has  no  chin,  and  the  upright  part  which  works  on 
the  skull  is  much  broader  than  in  that  found  at  Piltdown. 
According  to  Duckworth,  the  owner  of  the  Heidelberg 
jaw  must  have  been  similar  or  even  identical  with  the 
monkey-man  of  Java.  Most  anthropologists,  however, 
regard  this  Heidelberg  jaw  as  definitely  human,  and  as 
quite  distinct  from  the  Javan  monkey-man. 

310 


The  Geological  History  of  Man 

So  little  is  known  of  this  Heidelberg  man  that  very 
little  can  be  determined  as  to  his  affinities.  The  Neander- 
thal race  is,  however,  much  better  known.  It  is  named 
from  a  fragment  of  a  skull  found  in  a  cave  in  the  Neander- 
thal, near  Diisseldorf,  in  north-western  Germany,  but 
more  complete  knowledge  of  this  race  is  due  to  the  dis- 
covery of  whole  skeletons  in  the  south  of  France,  as  well 
as  to  the  skulls  found  at  Gibraltar,  and  at  Spy  in  Belgium. 


FIG.  29.— OUTLINE   OF  A   SKULL  OF   NEANDERTHAL   MAN. 

Note  the  prominent  brow-ridges,  the  absence  of  chin,  and  the 
flattened  top  of  the  skull. 

The  Neanderthal  race  had  large  teeth,  a  receding  forehead, 
with  strongly-raised  ridges  along  the  eyebrows,  and  a  very 
large  brain  (Fig.  29). 

Palaeolithic  man  certainly  included  members  of  more 
than  one  race.  According  to  Klaatsch,  Neanderthal  man 
was  descended  from  the  African  gorilla,  and  the  later 
Aurignac  man  was  the  descendant  of  the  orang-utan  of 
Malaysia.  This  view  is  rejected  by  most  authorities  as 


The  Geological  History  of  Man 

fantastic.  A  more  plausible  hypothesis  as  to  the  plurality 
of  Palaeolithic  races  is  based  on  the  belief  that  the 
Neanderthal  race  is  represented  at  the  present  time  by 
the  Australians ;  that  the  negroid  skeletons  known  as  the 
"  Grimaldi  "  race,  as  they  were  found  in  the  cave  of 
Grimaldi,  near  Mentone,  represent  the  bushmen  of  South 
Africa ;  and  that  some  of  the  Aurignacians  are  the 


FIG.  30.— RESTORED  OUTLINE  OF  THE  PILTDOWN  SKULL. 
(AFTER  SMITH  WOODWARD.) 

Note  the  absence  of  chin,  the  rounded  base  at  the  back,  and  the 
broad  ascending  portion  of  lower  jaw. 

ancestors  of  the  Eskimo.  According  to  that  view,  man- 
kind had  already  been  divided  in  Palaeolithic  time  into 
the  three  chief  subdivisions — Caucasian,  Mongolian,  and 
Negro. 

The  speculations  on  this  problem  have  been  greatly 
altered  by  the  discovery  in  1912  by  Mr.  Dawson  at  Pilt- 
down  in  Sussex  of  the  fragments  of  a  very  thick  and 
primitive  skull.  It  is  now  known  as  the  "  Piltdown  skull  " 

312 


The  Geological  History  of  Man 

(Eoanthropus  dawsoni),  and  has  been  described  by  Dr. 
Smith  Woodward.  The  skull,  which  was  perhaps  that 
of  a  woman,  has  revolutionized  opinion  as  to  the  geological 
history  and  antiquity  of  mankind.  Instead  of  the  forehead 
being  receding,  as  in  the  Neanderthal  man,  it  is  steep,  as 
in  an  average  European.  The  eyebrow  ridges  are  not 
thickened  and  projecting.  There  is  a  very  small  brain, 
which  was  larger  than  that  of  monkeys,  but  only  about 
two-thirds  the  size  of  that  of  modern  man  or  of  Neander- 


FIG.  31.— OUTLINE  OF  THE  SKULL  OF  A  MODERN  MAN 
(MONGOLIAN). 

Note  the  well-developed  chin  and  height  of  the  skull. 

thai  man.  The  jaw  is  like  that  of  the  chimpanzee.  The 
molar  teeth,  however,  are  distinctively  human,  though  the 
canines  are  larger,  as  in  monkeys.  According  to  Prof. 
Elliot  Smith,  the  brain  indicates  that  the  animal  probably 
had  no  power  of  distinct  speech. 

These  fragments  of  skull  were  found  associated  with 
Acheulian  and  Chellean  flint  implements,  so  that  it  was 
probably  contemporary  or  slightly  earlier  than  the 
Neanderthal  man ;  but  the  skull,  both  in  its  rounded  form 

313 


The  Geological  History  of  Man 

and  in  the  great  breadth  of  the  back  portion,  is  strikingly 
unlike  that  of  either  Neanderthal  or  modern  man,  and  the 
jaw  is  very  different  from  that  of  Heidelberg  man.  Still 
more  startling  is  the  fact  that  it  does  not,  like  the  monkey- 
man  of  Java,  tend  to  link  man  to  the  modern  anthropoid 
apes,  such  as  the  gorilla  or  orang-utan,  but  to  the  more 
primitive  round-headed  monkeys  of  the  Miocene  Period. 

Neanderthal  man  has  certain  gorilla-like  characters, 
but,  according  to  Dr.  Smith  Woodward,  these  features 
resulted  independently,  and  the  modern  anthropoid  ape, 
in  respect  to  these  characters,  is  described  as  having 
degenerated  from  the  earlier  monkeys ;  and  later  on  the 
same  features  were  produced  in  the  Neanderthal  race  by 
degeneration  from  a  round-skulled  race  of  men.  Dr. 
Smith  Woodward  interprets  the  evidence  of  the  Piltdown 
skull  as  showing  that  modern  man  did  not  pass  in  his 
evolution  through  the  condition  of  the  modern  anthropoid 
apes,  for  they  have  flattened  skulls.  Man  descended 
directly  from  the  Miocene  monkeys,  which  had  rounded 
skulls,  steep  foreheads,  a  wide-backed  head,  and  a  small 
brain.  The  race  of  Piltdown  retained  these  characters, 
but  an  off-branch  acquired  flattened  skulls  and  developed 
the  Neanderthal  type  of  man,  who,  though  his  general 
skull  characters  are  more  brutal,  appears  to  have  developed 
rapidly,  and  thus  acquired  a  much  larger  brain  than  the 
Piltdown  man.  Nevertheless,  the  Piltdown  race,  though 
a  slower,  was  a  surer,  development ;  it  advanced  into  the 
more  refined  and  more  cultured  Aurignacian  man,  from 
whom  modern  man  has  descended. 

The  question  as  to  the  relations  of  later  Palaeolithic 
man  to  the  three  main  modern  divisions  of  mankind  is 
still  merely  speculative.  The  evidence  is  suggestive,  but 
is  too  incomplete  to  be  anything  more.  The  races  of 
man  are  conveniently  divided  into  the  three  groups — the 
Negro  (including  the  Papuans  of  New  Guinea),  the 

314 


The  Geological  History  of  Man 

Mongolian  (including  the  American  Indians  and  Eskimo), 
and  the  Caucasians  (including  the  white  races,  the  dark 
Caucasians  of  northern  Africa  and  southern  Asia,  and  the 
aborigines  of  Australia). 

The  Palaeolithic  skeletons  which  have  been  found 
represent  three  distinct  types.  The  best  known  is  that 
named  Neanderthal  man  from  Neanderthal,  a  valley  near 
Diisseldorf  in  north  Germany.  The  top  of  a  skull  was 
found  there  in  1856,  and  Prof.  Huxley  was  at  first  almost 
alone  in  regarding  it  as  human ;  but  his  conclusion  has 
been  confirmed  by  the  discovery,  in  recent  years,  of  com- 
plete skeletons  of  this  race  in  the  south-west  of  France. 
The  Neanderthal  race  had  a  skull  with  a  low  crown,  a 
receding  forehead  ;  there  were  thick  bony  ridges  along  the 
eyebrows,  and  its  jaw  was  not  very  prominent.  Since 
these  were  regarded  as  primitive  characters,  the  Neander- 
thal was  named  Homo  primigenius.  According  to  a  long- 
popular  view  the  Australian  aborigines  are  his  living 
representatives.  This  appeared  probable  while  only  the 
front  of  the  cranium  was  known  ;  it  has  been  emphatically 
dismissed  by  most  recent  authorities  since  the  discovery 
of  more  complete  remains  of  the  Neanderthal  race. 
Prof.  Boule,  who  has  described  complete  Neanderthal 
skeletons  from  the  south-west  of  France,  has  compared 
them  in  great  detail  with  the  skeletons  of  the  Australians ; 
and  he  declares  that  the  difference  between  them  are 
fundamental.  He  says  that  "  the  Australian  type  has 
nothing  in  common  with  the  type  of  Neanderthal,  except 
a  small  number  of  localized  differences  in  the  frontal  and 
fronto-nasal  regions."4  In  contrast  to  these  slight  resem- 
blances in  the  forehead  and  nose,  there  are  important 
differences  in  the  rest  of  the  skull  and  throughout  the 
skeleton.  The  differences  between  the  skulls  of  the  two 
races,  Prof.  Boule  says,  "  leap  to  the  eye  and  have  no  need 
of  being  demonstrated."6  The  view  of  many  anthro- 

315 


The  Geological  History  of  Man 

pologists  that  the  Neanderthal  man  became  extinct  and  has 
no  living  representative  appears  therefore  highly  probable. 

The  two  skeletons  known  as  the  Grimaldi  race  are 
described  as  "  negroid  "  ;  but  the  view  that  the  race  was 
actually  Negro  has  been  questioned,  though  it  is  admitted 
that  the  characteristic  features  of  these  skeletons  are  more 
often  found  in  Negroes  than  in  any  other  modern  race. 
Moreover,  some  upper  Palaeolithic  drawings  represent 
people  with  the  most  conspicuous  physical  feature  of  the 
Bushman. 

It  would  appear,  therefore,  probable  that  some  of  the 
later  Palaeolithic  men  were  already  Negro,  or,  at  least, 
negroid  in  type.  Another  branch  of  the  human  race  had 
already  been  separated  from  this  negroid  stem  and  had 
acquired  several  Mongolian  characters  which  have  been 
recognized  in  Aurignacian  skeletons.  The  most  striking 
link  with  the  living  Mongolian  race  is  in  the  strong 
resemblance  of  upper  Palaeolithic  culture  and  modes  of 
life  to  those  of  the  modern  Eskimo.  It  has  often  been 
suggested,  as  by  Prof.  Boyd  Dawkins,  that  the  Eskimo 
are  the  direct  descendants  of  Palaeolithic  men;  and  the 
features  in  common  are  too  obvious  for  this  suggestion  to 
be  lightly  set  aside,  though  most  authorities  reject  it. 

The  Piltdown  man  and  his  contemporaries  lived  in  a 
comparatively  mild  climate  and  were  associated  with 
southern  animals.  It  was,  therefore,  quite  possible  for 
them  to  have  lived  at  that  time  in  south-eastern 
England  in  simple  bivouacs ;  but  after  the  disappearance 
of  the  old  elephant,  EUphas  antiquus,  the  climate  became 
colder,  and  the  reindeer,  mammoth  and  other  Arctic 
animals  ranged  in  western  Europe  as  far  south  as  England 
and  France ;  and  under  these  cold  conditions  man  was 
forced  to  shelter  in  caves.  He  appears  to  have  lived 
largely  upon  reindeer,  and  may  have  partially  domesticated 
them ;  but  with  the  return  of  warmer  weather  the  reindeer 
withdrew  to  the  north,  and  it  seems  not  at  all  improbable 


The  Geological   History  of  Man 

that  Palaeolithic  man  followed  them,  and  reached  Green- 
land either  across  land  which  has  sunk  below  the  North 
Atlantic  or  by  a  north-eastern  route  across  Asia  and 
Behring  Straits. 

Meanwhile,  another  section  of  Aurignacian  man  re- 
mained in  the  south,  and  with  the  advantage  of  improving 
climatic  conditions  developed  into  Azilian,  and  next  into 

Coin,  ca  5  tan  Mongo/ 

Grimalcfi  race    Aurignacian 


N«<^ro  Caucasan.          Mo 


1{ 
J{ 


Round  skulled  Honkny 


FIG.  32.  —  DIAGRAM  OF  EVOLUTION  FROM  THE  MIOCENE  MONKEYS. 

Neolithic  man  ;  and  the  Neolithic  European  was  a 
Caucasian.  The  probable  relations  of  the  three  chief 
types  of  mankind  to  their  fossil  predecessors  is  suggested 
in  the  diagram,  Fig.  32.  According  to  these  conclusions, 
which  have  been  suggested  by  the  discovery  of  the  Pilt- 
down  skull,  the  geological  history  of  man  is  confined  to 
the  Pleistocene  Period,  and  we  need  not  expect  human 
remains  in  the  Pliocene. 

317 


The  Geological  History  of  Man 

The  exact  age  of  the  Piltdown  gravel  is  not  certain,  but 
it  appears  to  be  post-glacial ;  for  the  valley  in  which  it 
occurs  has  been  deepened  very  little  since  the  time  of 
Piltdown  man,  whereas  the  valleys  of  the  boulder  clay 
districts  of  south-eastern  England  have  been  greatly 
enlarged,  and  some  of  them  entirely  made  since  glacial 
times.  The  claims  that  Palaeolithic  implements  and  the 
bones  of  modern  man  have  been  found  under  boulder 
clay  is  therefore  improbable.  In  1911  much  attention 
was  attracted  to  the  discovery  of  a  human  skeleton  at 
Ipswich  under  the  boulder  clay.  The  skeleton  was 
described  by  Dr.  Keith  as  belonging  to  quite  a  modern 
type,  and  the  belief  that  such  a  skeleton  was  pre-glacial 
was  considered  to  indicate  that  the  ancestry  of  man  must 
date  from  long  before  the  Glacial  Period.  If  the  Ipswich 
man  lived  before  the  deposition  of  the  boulder  clay,  he 
would  be  older  than  the  Piltdown  man.  From  the  arrange- 
ment of  the  bones  the  body  appears  to  have  been  buried 
in  the  cramped  position  used  at  Neolithic  burials;  and 
after  inspection  of  the  site  I  feel  bound  to  agree  with 
those  who  hold  that  it  was  a  Neolithic  or  later  tomb.  If 
the  skeleton  had  been  laid  naturally  in  the  position  where 
it  was  found  before  the  deposition  of  boulder  clays,  the 
agent  which  deposited  that  material  would  probably  have 
swept  it  away. 

Palaeolithic  man  was  especially  abundant  and  varied  in 
France.  His  remains  are  less  common  in  England,  and 
have  not  been  found  north  of  Yorkshire,  though  Neolithic 
man  not  only  lived  in  England,  but  was  widespread  in 
Scotland,  Denmark,  and  Scandinavia.  It  was,  therefore, 
thought  that  Palaeolithic  man  lived  in  England  while 
Scotland  was  still  covered  with  snow  and  ice.  Messrs. 
Bishop  and  Mann  have  recently  found  remains  belonging 
to  the  uppermost  division  of  Palaeolithic  time  in  the 
island  of  Oronsay,  off  western  Scotland.  Accosding 


The  Geological  History  of  Man 

to  Bishop  and  Mann,  the  level  of  the  land  in  Oronsay 
was  then  thirty  feet  lower  than  at  present ;  the  climate 
of  the  country  was  also  milder  than  at  the  date  of  the 
Roman  settlement  nearly  1,900  years  ago,  when  the  land 
appears  to  have  been  exactly  at  its  present  height. 
Hence  Scottish  Palaeolithic  man  apparently  lived  at  a 
time  when  the  islands  of  western  Scotland  were  smaller, 
and  the  bays  and  lochs  were  larger,  than  they  are  now. 
A  period  with  a  mild  climate  may  have  intervened  between 
the  cold  time  contemporary  with  the  earlier  Palaeolithic 
men  of  the  south  of  England  and  the  possible  re-increase 
of  the  glaciers  in  Neolithic  times.  The  geographical  and 
climatic  changes  both  indicate  that  the  Azilian  men 
entered  Scotland  at  least  tens  of  thousands  of  years  ago. 


NEOLITHIC  MAN. 

It  is  generally  believed  that  a  long  interval  of  time 
separated  Palaeolithic  and  Neolithic  man.  In  various 
localities,  as  in  south-western  France,  some  evidence  of 
an  apparently  intermediate  race  has  been  discovered ;  but 
it  is  generally  believed  that  Neolithic  man  is  not  the 
descendant  of  Palaeolithic  man  of  western  Europe. 
Neolithic  man  is  regarded  as  a  new  race  which  came 
in  and  repeopled  the  country  after  the  disappearance  of 
Palaeolithic  man. 

Neolithic  man  was  far  more  advanced  in  civilization 
than  his  predecessors.  His  polished  and  ground  stone 
tools  must  have  been  far  more  effective  than  the  jagged 
edges  of  the  chipped  Palaeolithic  implements.  He  was 
also  skilled  in  making  pottery ;  he  kept  domestic  animals 
and  was  an  agriculturist,  though  he  still  obtained  large 
supplies  of  food  from  fish  and  shell-fish.  He  had  not 
discovered  the  art  of  making  metal  tools,  though  he  used 
gold  for  ornaments.  The  gold  was  no  doubt  obtained 

319 


The  Geological   History  of  Man 

from  grains  in  the  river  gravels,  where  it  attracted  atten- 
tion by  its  beauty  and  the  ease  with  which  it  could  be 
worked  into  simple  designs. 

The  Neolithic  dwellings  were  often  built  for  shelter  on 
piles  in  lakes.  These  lake  villages  were  connected  to 
the  shore  by  drawbridges,  while  the  people  were  appar- 
ently expert  in  navigating  large  canoes  dug  out  of  single 
oak-trees. 

Neolithic  man  must  have  had  a  well-developed  religion, 
and  have  believed  in  the  immortality  of  the  soul ;  for  in 
many  cases  the  bodies  were  buried  reverently,  and  prob- 
ably with  elaborate  rites.  Tools  and  broken  pottery  were 
placed  with  the  corpse,  and  they  were  broken,  so  that  the 
spirits  of  these  implements  might  be  set  free,  and  accom- 
pany their  owner  to  the  spirit  world.  The  graves  were 
often  protected  by  a  mound  of  earth,  known  as  a 
"  barrow,"  or  by  a  stone  chamber ;  and  these  monuments 
passed  into  more  elaborate  structures,  such  as  Stone- 
henge,  which  was  probably  erected  in  the  Bronze  Age 
as  the  temple  of  a  race  of  sun  worshippers. 


THE  BRONZE  AGE. 

The  great  advance  made  at  the  end  of  Neolithic  time 
was  the  discovery  how  to  make  cutting  tools  of  metal. 
Native  copper  was  probably  found  early — though  it  is  not 
abundant  in  Europe — and  it  is  easily  worked;  but  it  is 
too  soft  to  be  of  much  value  for  tools.  The  discovery, 
however,  of  hardening  copper  by  melting  tin  with  it,  and 
thus  producing  the  metal  called  bronze,  gave  man  a  metal 
of  great  usefulness.  As  soon  as  the  descendants  of  Neo- 
lithic man  were  armed  with  this  new  material  they  made 
rapid  progress  in  culture.  Bronze  tools  are  found  in 
older  deposits  than  those  containing  tools  of  iron,  and 
archaeologists  therefore  usually  consider  that  man  used 

320 


The  Geological  History  of  Man 

bronze  long  before  his  discovery  of  the  use  of  iron.  But 
this  conclusion  is  attended  with  a  serious  difficulty. 

The  preparation  of  bronze  is  a  difficult  process.  Tin 
is  only  found  in  any  quantity  in  a  few  widely  scattered 
localities.  In  prehistoric  and  early  historic  times  Corn- 
wall was  the  chief  source  of  supply  in  Europe,  and 
merchants  from  the  Mediterranean  came  to  England  to 
trade  for  tin.  Ample  supplies  of  copper  ores  were  ob- 
tained from  the  Mediterranean  countries,  as  Spain, 
Cyprus,  and  Asia  Minor.  The  art  of  forming  a  useful 
alloy  of  tin  and  copper  is  so  difficult  that  it  appears 
metallurgically  almost  impossible  that  men  could  have 
prepared  bronze  until  they  had  gained  considerable  ex- 
perience in  working  more  easily  prepared  metals,  such 
as  iron.  It  has  been  suggested  that  primitive  man  dis- 
covered the  manufacture  of  bronze  accidentally — perhaps 
while  melting  copper  on  a  river-bed  it  may  have  become 
combined  with  some  tin — just  as  the  story  is  told  of  the 
Phoenician  discovery  of  glass.  The  chances  of  producing 
a  useful  bronze  under  such  conditions  would  appear  to  be 
so  small  that  its  production  would  have  to  be  regarded  as 
little  less  than  a  metallurgical  miracle.  Tin  ore  is  found 
in  the  form  of  tin  oxide  (tin  combined  with  oxygen),  and 
the  chief  difficulty  in  preparing  bronze  castings  is  that  the 
presence  of  a  minute  trace  of  tin  oxide  causes  the  bronze 
to  be  so  brittle  as  to  be  practically  useless.  The  use  of 
bronze  for  cannon  was  largely  abandoned,  in  spite  of  its 
many  suitable  qualities,  owing  to  the  uncertainties  in  the 
strength  of  the  bronze,  due  to  traces  of  tin  oxide. 

Iron  is  much  more  easily  smelted  on  a  small  scale  than 
bronze.  Grains  of  iron  oxide  are  very  widely  distributed, 
and  in  arid  areas  they  attract  attention  by  their  heaviness 
and  metallic  aspect.  When  mixed  with  charcoal  in  a 
briskly  blown  fire,  the  ore  is  reduced,  and  an  excellent 
mild  steel  is  produced.  The  formation  of  such  particles 

321  x 


The  Geological  History  of  Man 

of  iron  would  be  easily  noticed,  an  by  heating  and 
hammering  them  they  would  be  welded  into  larger  pieces. 
The  preparation  of  iron  by  the  Negroes  in  Africa  is  a  far 
simpler  process  than  the  manufacture  of  bronze.  It  is 
therefore  probable  that  men  would  have  made  iron  tools 
first,  and  that  the  use  of  iron  preceded  that  of  bronze. 
Bronze  tools,  however,  are  found  in  Europe  earlier  than 
those  of  iron,  but  their  earlier  presence  may  be  explained 
by  the  readiness  with  which  iron  tools  would  perish  from 
rust.  Bronze  tools  are  preserved  in  beds  from  which  iron 
implements  would  have  completely  disappeared. 

This  explanation  of  the  absence  of  iron  from  Bronze 
Age  graves  is  not  satisfactory,  for  if  iron  had  been  present 
and  removed,  the  rust  should  sometimes  have  remained 
as  a  stain,  or  even  as  a  cement  fastening  together  some  of 
the  adjacent  grains  of  earth.  Moreover,  it  is  clear  that  in 
western  Europe  the  people  of  the  Bronze  Age  immediately 
succeeded  those  of  the  later  Stone  Age,  for  the  early 
bronze  implements  are  copies  of  stone  tools.  The  conflict 
of  the  metallurgical  and  archaeological  arguments  probably 
admits  of  a  geographical  explanation.  Grains  of  iron  ore 
in  sands  and  gravels  are  conspicuous  only  in  hot,  arid 
climates,  such  as  tropical  Africa ;  and  it  is  probable  that 
iron-working  was  invented  there  before  the  Bronze  Age 
in  Europe.  The  inhabitants  of  the  moister  climates  of 
the  Mediterranean  and  Europe  had  no  such  easily  found 
supply  of  iron.  Some  conspicuous  ores  yielded  tin  and 
copper,  and  some  ingenious  smith  who  had  learnt  iron- 
working  in  tropical  Africa  may  have  combined  them,  and 
thus  obtained  bronze.  It  was  probably  not  till  after  this 
discovery  that  the  European  aborigines  found  that  iron 
could  be  extracted  from  the  earthy  ores6  of  the  northern 
moorlands,  as  well  as  from  the  glistening  grains  in  the 
African  sands. 

In  Africa  there  was  no  Bronze  Age;  the  use  of  iron 

322 


The  Geological  History  of  Man 

directly  succeeded  that  of  stone.  But  in  Europe  men 
used  bronze  until  they  found  the  less  easily  discovered 
ores  of  iron,  and  the  adoption  of  iron  marked  the  passage 
from  the  domain  of  geology  to  that  of  history. 

1  W.  Cunnington,  "The  Authenticity  of  Plateau  Man,"  Natural 
Science,  1897,  vol.  xi.,  pp.  327,  333. 

2  The  only  one  of  the  deposits  which  has  been  claimed  as  Red 
Crag  above  an  implement  band  which  I  have  examined  seems  to  me 
to  consist  of  crag  material  which  has  been  redeposited  in  Glacial 
times.    The  flints  at  its  base  are  therefore  later  than  the  Red  Crag, 
but   I   understand  that  in  other  pits  the   Red  Crag  above  these 
implement-bearing  gravels  is  still  in  its  original  position. 

3  Quart.  Journ.  Geol.  Soc.,  1860,  vol.  xxxvi.,  p.  527. 

4  Boule,  1913,  p.  233.  6  Ibid.,  p.  231. 

5  Iron-working  in  Europe  may  have  begun  with  such  metallic- 
looking  ores  as  those  of  Elba. 


323 


INDEX 


(Localities  are  grouped  under  England,  Scotland,  and  the  Chief  Continents.} 


ACERATHERIUM,  273 

./Epyornis,  265 

Aerolites,  40 

Africa,  214,  322  ;  Atlas  Mountains,  221 ; 
Egypt,  273  ;  Fayum,  275  ;  German 
East  Africa,  259  ;  Great  Rift  Valley, 
128,  154 ;  Madagascar,  265 ;  Nile, 
170,  171,  245 ;  South,  260,  269 ; 
Teleki  Volcano,  127 

Agassiz,  Louis,  223,  224 

Agglomerate,  120 

Alps,  28,  32,  221 

America,  North  :  Boston,  46  ;  British 
Columbia,  104 ;  California,  104 ; 
Canada,  69,  231  ;  Colorado,  255 ; 
Dakota,  274;  Devil's  Cany  on,  45,  138, 
140 ;  Greenland,  45,  231  ;  Labrador, 
231  ;  Mexico,  45  ;  Mississippi,  24, 
171  ;  Nebraska,  263  ;  New  York,  258  ; 
Oregon,  45 ;  Pennsylvania,  242  ; 
Rocky  Mountains,  33,  257 ;  San 
Francisco,  earthquake,  110-112;  St. 
Elias  Mountain,  119 ;  Texas,  185, 
253 1  Wyoming,  271 ;  Yellowstone 
Park,  124 

America,  South  :  Andes,  101,  153,  173, 
206,  222,  282  ;  Argentine,  282  ;  Cara- 
cas earthquake,  130  ;  Ecuador,  281  ; 
Last  Hope  Inlet,  282 

Amia,  244 

Ammonites,  215,  291 

Amphibians,  209,  245,  246,  249 

Andes.     See  South  America 

Andesite,  130 

Andrews,  C.  W.,  275,  283 

Antrim,  221 

Arago,  46 

Archaeopteryx,  216,  264 

Armorican  Mountains,  209 

Arrhenius,  46 

Arsinotherium,  273 

Asia:  Assam,  earthquake  of,  106-108; 
Bengal,  Bay  of,  162;  Bromo,  Java, 
133 ;  Caspian,  104  ;  Caucasus,  222  ; 
Cyprus,  279,  287  ;  Everest  Mountain, 
157;  Ganges,  162;  Garo  Hills,  108; 
Himalaya,  28,  159,  161,  162, 173,  221, 


222  ;  India,  69,  101,  211,  214,  254,  261, 
265  ;  Japan,  99,  101,  114  ;  Java,  133, 
309,  314 ;  Khasi  Hills,  106,  107 ; 
Krakatoa,  102,  120,  135-138  ;  Malay- 
sia, 104 ;  Philippine  Islands,  104 ; 
Siberia,  271  ;  Siwalik  Hills,  251,  261  ; 
Sunda,  Straits  of,  120, 135 ;  Tokio,  99 
Atikokania  lawsoni,  192 
Aurignacian  man,  308,  311,  317 
Australia,  101,  193,  209,  211,  216,  254, 
265,  268,  269,  291,  298  ;  Central 
Australia,  137,  215,  290  ;  Lake  Eyre, 
250  ;  Melbourne,  100 ;  Mt.  Dyrring, 
43,  46  ;  New  South  Wales,  43,  212  ; 
Queensland,  208,  245  ;  rivers  in,  153  ; 
South  Australia,  48 ;  Sydney,  212  ; 
Victoria,  270 

Ball,  Sir  Robert,  38 

Barr,  A.,  177,  294,  295,  297 

Beaches,  raised,  32 

Beadnell,  H.  J.  L. ,  273 

Beaumont,  Elie  de,  144 

Becker,  G.  F.,  174,  176,  178,  179,  185 

Belemnites,  215,  291 

Beltina,  192 

Birds,  origin  of,  263-265 

Bishop  and  Mann,  318 

Boltwood.  B.  B.,  185 

Bonney,  T.  G.,  71,  72,  225 

Boulder  clay,  224,  226-230 

Boule,  Marcellin,  305,  315 

British  earthquakes,  115-117 

British    Isles,   volcanoes  in.    131.     See 

also  England 
Brontosatirus,  257 
Bronze  Age,  320-323 
Broom,  R.,  266 
Bruckner,  E.  E.  M.,  228 
Brun,  A.,  132,  133 
Bryozoa,  222 
Buch,  L.  von,  27 
Bunsen,  77,  82,  123 
Burnett,  Bishop,  18 

Calamites,  211 
Caledonian  Trend,  205 


324 


Index 


Cambrian  System,  198 

Campsognathus,  255 

Carbohydrates,  236 

Carboniferous  System,  206-210 

Carnegie,  Andrew,  257 

Catalyser,  237 

Catastrophists,  22 

Celsius,  26 

Cephalaspis,  243 

Ceratodus,  245,  246 

Ceratosaurus,  257 

Cetiosaurus,  257 

Chalk,  218 

Chamberlin,  T.  C,  44,  45 

Chlorine  in  sea-salt,  185 

Clays,  64  ;  red  clay,  65 

Cleistopora,  207 

Club-mosses,  giant,  211 

Coal  Measures,  British,  159 

Coal-seams,  208 

Coecilians,  248 

Coenolestes,  281 

Cole,  G.  A.  J.,  86 

Coleman,  A.  P.,  194 

Comets,  41,  46 

Comstock  Mine,  147 

Contact  metamorphism,  68 

Conway,  Sir  W.  Martin,  227 

Cook,  Captain,  265 

Cornstones,  202 

Cosmogonists,  18 

Cowper,  W.,  101-102,  187-188 

Crag-beds,  223 

Craters  and  caldrons,  125  ;  non-vol- 
canic craters ;  138 

Cretaceous  System,  217 

Crinoids,  201 

Crocodiles,  251 

Cryptobranchus  scheuchzcri,  supposed 
fossil  man,  247,  300 

Crystals,  growth  of,  285 

Cunnington,  W.,  304,  305 

Cuvier,  240,  241 

Cycads,  214 

Cynodonts,  267 

Cynognathus,  253,  254 

Daly,  R.  A.,  86,  196 

Dana,  J.  D.,  143 

Dante,  17 

Durwin,  Sir  George,  42,  83,  167 

Darwinism,  284 

Dawkins,  W.  Boyd,  308,  316 

Dawn  of  Life,  189-197 

Deer,  great  Irish,  272 

Deluc,  17 

Denudation,  processes  of,  90 

Devil's  Canyon,  45,  138,  140,  141 

Devonian  System,  202-206 

Diastase,  237 

Dicynodons,  254,  261 


Dimetrodon,  254 
Dimorphodon,  262 
Dinoceras,  274 
Dinosaurs,  252,  255,  263 
Dinotherium,  278 
Diplodocus,  257,  295 
Diprotodon,  269 
Dipterus,  246 
Dolichosoma,  248 
Dollo,  L.,  256 
Drepanaspis,  243 
Duck-billed  Platypus,  254 
Dust,  volcanic,  120 
Dutton,  C.  E.,  161 

Earth,  age  of,  166-188  ;  birth  of,  35-51  ; 
geology  of  inner,  52-60  ;  materials  of, 
6 1  ;  materials  of  crust  of,  61-87  ;  move- 
ments of  crust  of,  88-93  ;  tremors,  99  ; 
weight  of,  53 

Earthquakes,  98-117 ;  cause  of,  58 ; 
depth  of,  56-57 ;  region  of,  104-105  ; 
speed  of,  58-60 ;  sound,  100 ;  wave,  100 

Edentates,  281 

Ekholm,  Nils,  182 

Elasmotherium,  274 

Elephant,  274 

Elginia,  253 

England,  230;  Bala,  200;  Brixham 
Cave,  302 ;  Cheshire,  214  ;  Cornwall, 
222 ;  Croydon,  290  ;  Devon,  202  ; 
Dorsetshire,  216,  262  ;  Downs,  27  ; 
East  Anglia,  27,  116,  172 ;  Essex,  93, 
117,  222,  226,  230  ;  Falmouth,  170  ; 
Felixstowe,  184  :  Flamborough  Head, 
226  ;  Flintshire,  206  ;  Ightham,  303 ; 
Ipswich,  318  ;  Kent,  27,  167,  217 ; 
London,  114  ;  Maidstone,  256  ;  Moel 
Tryfaen,  225  ;  Norfolk,  98,  222  ; 
Oxfordshire,  216 ;  Pennine,  210 ; 
Sheppey,  220,  250  ;  Skiddaw,  67,  68  ; 
Snowdon,  200 ;  Suffolk,  169,  122, 
306,  307  ;  Thames  River  and  Valley  ; 
24,  27,  175,  219,  274,  278 ;  Wor- 
cester, 214 

Eocene,  220,  270,  280 

Eoliths,  303-306 

Eozoic  Era,  190-197 

Eozoon,  191 

Estheria,  214 

Europe,  213,  279,  323 ;  Abbeville,  301 ; 
Alps,  228  ;  Baltic,  26 ;  Black  Sea, 
229,  Bothnia,  Gulf  of,  27  ;  Crete,  279, 
287  ;  Crimea,  218  ;  Dordogne,  278  ; 
Eifel,  243;  Etna,  65,  101-102,  131  ; 
Grimaldi  Cave,  312,  316 ;  Ischia, 
102-104  ;  Jura  Mountains,  28,  215  ; 
Lisbon  earthquake,  105,  109,  no; 
Malta,  138,  279,  287  ;  Messina,  101 ; 
Mont  Blanc,  121  ;  Norway,  27,  32, 
185  ;  Oesel,  Island  of,  242  ;  Pozzuoli, 


325 


Index 


28,  30 ;  Pyrenees,  222,  281 ;  Russia, 
212,  254 ;  St.  Gothard,  72,  147 ; 
Scandinavia,  95,  173,  193,  200,  231, 
280 ;  Simplon  Tunnel,  147  ;  Solen- 
hofen,  264 ;  Somrae  Valley,  301  ; 
Spitsbergen,  194,  203,227;  Sweden, 
26,  231  ;  Switzerland,  224;  Tyrol 
(Innsbruck),  32,  41,  228  ;  Vesuvius, 
104,  120,  133,  191 

Eurypholis,  245 

Eurypterids,  240 

Evans,  Sir  John,  301,  303 

Evolution,  cause  of,  284-293 

Extinct  animals,  size  of,  294-299 

Faults,  96,  97,  114,  134 

Felspar,  67 

Fiord  areas,  151 

Fireclay,  208 

Fish,  armoured,  244,  245 ;  bony,  244  ; 

Era  of  a  Palaeozoic,  191-212 
Fitton,  21 
Fletcher,  L.,  54 
Fluid,  definition  of,  156 
Fold  and  Faults,  94-97 
Folds,  95,  96 
Fossil,   63 ;    absence  of    Eozoic,    194- 

197  ;  interpretation  of,  239-248  ;  use 

of,  189,  190 

Ganoids,  244 

Gaskell,  240 

Gavials,  251 

Geer,  Baron  G.  J.  de,  231 

Geikie,  Sir  Archibald,  170,  203 

Geikie,  James,  225,  228 

Geological  Society,  London,  20 

Geology,  physical,  88 

Geysers,  121-124 

Gigantosaurus,  259 

Gilbert,  G.  K.,  139,  140 

Glaciation  Periods,  228-232 

Glossopteris  Flora,  211,  212 

Glyptodon,  282 

Gneiss,  68,  69 

Gneiss,  Moine,  86 

Goethe,  19,  20,  30 

Gondwanaland,  212,  215,  266 

Granite,  78 

Graptolites,  199 

Great  Rift  Valley,  128,  154 

Green,  Lowthian,  132,  133 

Gunn,  Marcus,  242 

Halley,  174 
Hallopus,  255 
Harker,  H.,  87 
Harrison,  Benjamin,  303 
Haughton,  S.,  170 
Hawaii,  132 
Hayford,  J.  F.,  163,  164 


Hecker,  O.,  162,  163 
Heidelberg  man,  310,  314 
Helmholtz,  38 
Himalaya.    See  Asia 
Hinde,  G.  J.,  192 
Hipparion,  271 
Hogbom,  A.  G.,  206 
Holarctica,  280 
Holland,  SirT.  H.,  257 
Holmes,  A.  R.,  179 
Holoptychius,  247 
Horns,  272 
Horse,  descent  of,  271 
Huggins,  Sir  William,  37,  43 
Hutton,  20,  23,  25 
Huxley,  234,  248,  251,  315 
Hyracotherium,  271 

Ice-scratched  boulders,  224 
Ichthyosaurians,  216,  241,  259,  260,  287 
Iguanodon,  218,  250,  256 
Implements,  Sub-Crag,  306 
Invertebrates,  239 
Ipswich  man,  318 
Isochlors,  176 
Isostasy,  161-165 

Jardine,  Sir  William,  211 
Jensen,  H.  I.,  84 
Johnson,  W.  D.,  139 
Johnston- Lavis,  H.  J.,  85,  104 
Joly,  J.,  56,  I4S.   146.   147,  148,   174, 
178,  179 

iudd,  J.  W.,  137,  138 
upiter,  Temple  of,  29,  94 
uras,  28 
Jurassic  System,  215 

Kainozoic  Era,  219-232 
Kelvin,  Lord,  179-183,  186 
Kent  Cave,  302 
Kilauea,  132,  133,  134 

Labyrinthodonts,  247 

Lamplugh,  G.  W.,  229,  230 

Lampreys,  248 

Land  forms,  90 

Lankester,  Sir  Ray,  277,  296,  306 

Laplace,  36-39,  42,  43,  54 

Lapworth,  C.,  71 

Lawson,  A.  C.,  no 

Lemberg,  85 

Leptolepis,  244 

Lewis,  Carvell,  226 

Limestones,      65  ;     Cambrian,     197  ; 

Eozoic,  196 ;  Wenlock,  201 
Liparite,  77 
Lister,  20 

Lockyer,  Sir  J.  Norman,  40,  41 
Loewinsson-Lessing,  F.,  84 
Lonsdale,  202 


326 


Index 


Lubbock,  Sir  John,  303 

Lungfish,  208 

Lycosaurus,  254 

Lyell,  21,  23,  25,  26,  27,  31,  34,  220 

Macculloch,  70 

Macrauchenia,  288 

Mammals,  evolution  of,  166-283 

Mammoth,  277,  278 

Man,  Geologic  history  of,  300-323  ; 
Neolithic,  319,  320;  Palaeolithic, 
307-319 

Marsh,  O.  C,  257,  258 

Marsupials,  268 

Martin,  C.  J.,  292 

Mastodon,  275,  277 

Mastodonsaurus,  247,  248 

McGee,  170,  174 

Megalania,  250 

Megalosaurus,  256 

Megatherium,  282 

Mesohippus,  271 

Mesozoic  Era,  213 

Metamorphism,  selective,  71 

Meteorites,  40-47,  52,  54,  139,  140 

Meteoritic  theory,  40-42,  44,  45 

Michel-Levy,  A.,  85,  86 

Mill,  H.  RM  98 

Millstone  Grit,  208 

Milne,  J.,  60,  99,  104 

Mingaye,  43 

Miocene,  220,  280,  317 

Moa,  285 

Moeritherium,  275 

Mole,  Australian,  241 

Monotremes,  254,  268 

Moraines,  224,  226 

Mosasaurus,  250 

Moulton,  39 

Mountains  :  Appalachian,  143 ;  defined, 
142 ;  distribution  of,  148-151  ;  for- 
mation of,  142  ;  how  made,  154  ; 
limestone,  206 ;  rootless,  149 ;  up- 
holding of,  155-165 

Mount  Dyrring  meteorites,  43,  46 

Mud,  volcanic,  120 

Murchison,  Sir  R.  I.,  70,  71,  210 

Neanderthal,  311,  313,  314 

Nebular  theory  and  Nebulae,  35-39,  41, 

Neomylodon,  282 
Neptunists,  19,  21 
New  Red  Sandstone,  214,  244,  251,  252, 

25S.  259 

Newton,  H.  A.,  46 
Newton,  Sir  Isaac,  53 
New  Zealand,  265  ;  North  Island,  124  ; 

Rotomahana,    134 ;    Tarawera,    102, 

120,  124,  134,  135 ;  Waimanga,  124 ; 

Wairakei,  124 


Norwich  Crag,  306 
Notharctus,  241 

Oldham,  R.  D.,  106,  107 

Old  Red  Sandstone,  202-205,  243,  244 

Olenellus,  195,  198 

Oligocene,  221 

Onohippidium,  283 

Oolitic  beds,  214 

Oozes,  65,  66 

Ordovician  System,  199 

Origin  of  life,  233-238 

Osborn,  H.  F.,  258,  259 

Ostracodermi,  242,  243 

Owen,  265 

Palaeomastodon,  273,  275 

Palceospondylus  gunni,  242 

Palseotherium,  273 

Pariasaurus,  253 

Penck,  A.,  228 

Penrith  Sandstone,  210 

Permian  System,  210 

Perry,  J.,  181,  182 

Perthes,  Bouchtr  de,  301 

Phenacodus,  271 

Phillips,  John,  169 

Pickering,  G.  F.,  45 

Piltdown    man,    310,    312,    314,    316, 

.318 

Pithecanthropus,  309 
Planetesimals,  44 
Playfair,  28 

Pleistocene,  221,  281,  307,  309 
Plenum,  Meteoritic,  47 
Plesiosaurs,  260,  261 
Pliocene,  229,  280,  307 
Plutonists,  19,  21 
Potholes,  92 
Poulton,  E.  B.,  187 
Prestwich,  Sir  Joseph,  301,  303 
Protein,  237,  238 
Proterosaurus,  249 
Protobion,  236 
Protoplasm,  235 
Protopterus,  245,  246 
Pteranodon,  263,  296 
Pterichthys,  243 
Pterodactyl,  283 
Pterosauria,  262 
Pumice,  74 
Purbeck  beds,  216 

Radium,  55,  56,  145-148,  183 

Rays  (fish),  243 

Red  Crag,  169,  184,  306,  323 

Reptiles,    Ancient,    249-1263;    Era    of 

Mesozoic,  213-218      ojxj.d  ; 
Rhinoceros,  273  ;  Woolly,  274 
Rhynchocephalia,  250,  251,  254 
Roberts,  I.,  37 


327 


Index 


Roc,  245 

Rocks :  acid,  75,  76  ;  alkali,  77  ; 
archean,  86  ;  assimilation  in,  77, 
84 ;  basic,  75,  76 ;  classification  of, 
63 ;  classification  of  igneous,  72 ; 
differentiation  in,  77,  82 ;  Eozoic, 
190-193  ;  foliated,  68  ;  hybrid,  87  ; 
igneous,  72  ;  metamorphic,  66  ; 
Plutonic,  74  ;  primary,  62-63  ; 
secondary,  63  ;  sedimentary,  86 

Rogers,  H.  D.,  158 

Rosse,  Lord,  37 

Rowe,  A.  W.,  286 

Salamanders,  247 

Salisbury,  Lord,  181 

Sandstones,  64 

Sandwich  Islands,  128 

Schafer,  Sir  Edward,  238 

Scheiner,  43 

Schist,  67,  69 

Schwarz,  E.  H.  L. ,  46 

Scoria,  120 

Scotland,  32,  95,  193,  230,  254  ;  Ayr- 
shire, 83,  200;  Broom,  Loch,  117; 
Crieff,  117 ;  Elgin,  249,  251,  253 ; 
Glasgow,  n6,  117;  Grampians,  53; 
Highlands,  69,  71,  86,  149,  205  ; 
Inverness,  116;  Lanarkshire,  243; 
Lomond,  Loch,  205;  Mull,  Isle  of, 
221 ;  Ochil  Hills,  206  ;  Oronsay,  318  ; 
Perthshire,  20  ;  Schiehallion,  53  ; 
Skye,  Isle  of,  221  ;  Stirling,  207 

Sea,  level  of,  32  ;  urchins,  286 

Sedgwick.  Adam,  198 

Sedimentation,  160 

See  T.J.  J.,39,47 

Sharks,  242  ;  primitive,  201 

Shell-beds,  British,  222 

Shingle  rivers,  284 

Shooting  stars,  40 

Silurian  System,  200 

Sinter,  141 

Smith,  Elliot,  313 

Smith,  William,  20,  189 

Snakes,  290 

Sodium  in  sea,  175,  177 

Solar  System,  35,  36,  39,  46,  179 

Solidification,  Sequence  of,  80 

Sorby,  W.  C.,  172 

Spectrum,  42 

Steam,  Volcanic,  131,  132 

Stegocephalia,  247 

Stegosaurus,  256 


Stonesfield  Slate,  216 

Strachey,  20 

Stratum,  63 

Strutt,  Prof.  R.  J.,  55,  56,  145,  146,  182, 

184 

Submerged  forests,  28 
Subsidence,  160 
Suess,  E. ,  30,  31,  34,  87 

Table  Mountain  Sandstone,  192 

Tail,  180,  181,  186 

Taylor,  Griffith,  224 

Tethys— the  early  Mediterranean  Sea, 

215 

Tetrabelodon,  276 
Thecodonts,  267 
Theophrastus,  17 
Thermo-Metamorphism,  69 
Theromorphs,  252-254,  261 
Titanotherium,  274 
Torridon  Sandstone,  184,  192,  193 
Tortoises,  261 
Transgressions,  Marine,  33 
Trees,  Size  of,  296 
Triassic  System,  214 
Triceratops,  255,  257 
Trigonia,  291 
Trilobites,  198 
Tritylodons,  254 
Turtles,  261 
Tyrrell,  G.  W.,  83 

Uniformitarians,  22,  23,  24 
Uranium,  145 

Vertebrates,  240,  241 
Vinci,  Leonardo  da,  17 
Volcanoes,  118-141 

Walcott,  C.  D.,  192 

Walker,  G.,  235 

Water,  Level  of,  32 

Weal  den  beds,  217 

West  Indies :  Anguilla,  296 ;  St.  Vin- 
cent, 128-130;  Mont  Pelee,  100, 
128-130 

Wiechert,  60 

Winchell,  A.,  169 

Woodward,  A.  Smith,   240,  285,  286, 

314 
Bright,  Joseph,  225 

Zangwill,  I.,  18 
Zaphrentis,  209 


31 
Wri 


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